Are 21 inch thighs big or not? It depends if you know where to measure your thighs.
For example, if you're measuring your legs near your knee, then a 21 inch thigh circumference is actually quite large. However, this isn't the correct way to measure your thighs.
You need to measure around the mid or upper thigh to get an accurate reading.
In this case, 21 inch legs are a very average measurement—for both men and women. This 21 inch average is actually based on US anthropometric data that examined over 8,000 participants, so you know that it's pretty accurate.
See How Your Thighs Stack Up:
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What do 21 inch thighs look like?
What do 21 inch thighs look like? It depends on your height, primarily, but also on your body composition.
Let me explain. If you're 5'1″, then 21 inch thighs will look less slim on your physique than that of someone who's 5'9″. This is simply because the shorter person has the same amount of thigh mass spread over a much smaller surface area, which makes their thighs look bulkier.
Some women can be insecure about this, believing that their thighs are too big when, in fact, according to science and most people's opinion, their legs are a perfectly normal size.
The point is that a 21 thigh circumference will look fairly slim at just about all heights and will look slimmer the taller that you are (or, more accurately, the longer that your femurs are).
If you're a man with 21 inch thighs, then they can actually look quite muscular if you have a good quad sweep, which is to say decently-developed thighs.
Of course, some men have 21 inch legs without lifting weights. However, their 21 inch quads may not look as big as your 21 inch quads if they have longer legs. This is because the same amount of muscle is stretched over a longer surface area.
Are 21 inch thighs big for a woman?
It's very normal for females to have 21 inch thighs. The one exception to this is elderly women (aged 70+), who typically have thinner thighs than younger women.
Your height and body composition do, of course, affect how slim your thighs look.
All else being equal, 21 in thighs will look slimmer on a taller frame than on a shorter frame. As mentioned above, this is simply because the same amount of thigh mass is effectively stretched over a longer surface area when you’re tall.
Additionally, the more toned that your 21″ thighs are, the more aesthetic they will look in some people's eyes.
Of course, if you have really well-developed quadriceps, then that can actually make your legs look bigger than they are—but only when the flesh is visible (you can’t really assess someone's leg development when they're wearing pants).
If you believe that your 21 inch or 21.5 inch thighs are too big for your height, then you might have genetically muscular legs. For example, you might have the same amount of thigh fat as your friends, but because of your genetics, you just so happen to have considerably more muscle mass too.
Are 21 inch quads good for men?
21 inch quads are a little on the small side for a man who lifts weights regularly. The reason for this is that, based on the data, most men in the general population have 21 inch quads.
Now, if you have lean 21 inch quads, then your thighs will obviously look a lot bigger than they actually are, and so while your thigh circumference may be average, your actual quadriceps lean mass would be above average.
I've seen several "measurement videos," for example, where the guy had 21 inch quads or thereabouts, and yet his thighs looked much bigger because they had good definition.
Additionally, the shorter that you are—specifically, the shorter that your femurs are—the bigger that your 21″ quads will look.
On the other hand, if you're tall, especially over 6 feet, then 21 in quads may actually look a bit skinny, especially if you don't lift weights.
The reason for this is that when your femurs are long, your muscle mass is spread or "stretched" over a longer surface area than when your femurs are short.
How can you grow your 21 inch thighs?
If you want to grow your 21 inch thigh circumference, then make sure that your diet is sufficient to support your training. Otherwise, your efforts in the squat rack will be in vain.
Get at least 20g of protein per serving (likely more if you're eating a mixed meal containing carbs and fats) so that you stimulate muscle protein synthesis and kickstart hypertrophy. 
You also need to consume enough fat so that you can achieve and maintain optimal hormone production.  Having healthy testosterone levels will enable you to improve the appearance of your quads (and the rest of your physique, for that matter) by building muscle and dropping body fat.
Your carbohydrate intake should be scaled to your training intensity and activity level. As long as you have enough carbs to fuel your workouts and trigger modest weight gain, you're likely eating enough.
On the training side of things, make sure to pick a few core leg exercises and stick with them. No, you don't have to squat, but the back squat is a very good exercise.
Performing the same exercises week in, week out enables you to gain strength and progressively overload your quads with more tension to make them grow.
On the other hand, if you keep changing your exercises and just train merely for the pump, then you likely won't see much in the way of quad growth. This is because when you alter your exercise selection, you typically also reduce the resistance initially, which can ruin your progress.
How can you slim your 21 inch legs?
When you lose weight in general, all of your circumference measurements will become slimmer—including your thighs.
Increasing your activity levels, avoiding junk food, and eating lower-calorie foods are all proven ways to shed the pounds. 
Just don't skimp on your protein.
Many people greatly desire lean, toned thighs, which you can only sculpt by having sufficient levels of muscle mass, which, in turn, you can only achieve by consuming enough protein.
The verdict: Is it good to have a 21 inch thigh size?
If you have a 21 inch thigh size, then—regardless of your gender—you have nothing to worry about because 21 inch thighs are a completely normal size.
The one exception to this—and a very rare scenario—is when your 21 inch legs may actually be too small and a sign that you're underweight. For example, if you're really tall, let's say 6'6″, and still only have 21″ thighs, then that's likely an indication that you have a low BMI.
For most people, however, 21 inch quads are a sign that your body weight is normal and healthy.
- Nieman, D. (2009). Ingested protein dose response of muscle and albumin protein synthesis after resistance exercise in young men. Yearbook of Sports Medicine, 2009, 144–145. https://doi.org/10.1016/s0162-0908(09)79472-3
- Eastwood, P. (2019, January 18). Rising Role Of Fat Reveals Importance In Strength And Performance! Bodybuilding.Com. https://www.bodybuilding.com/fun/fats-hormone-production-role.htm
- NHS website. (2021, November 25). 12 tips to help you lose weight. Nhs.Uk. https://www.nhs.uk/live-well/healthy-weight/managing-your-weight/12-tips-to-help-you-lose-weight/