Crunches is the go-to exercise for people wishing to reduce the belly. But wouldn't training the belly muscles actually increase the belly size? Given the fact that fat spot reduction is a myth, wouldn't we end with more mass by performing crunches (or similar exercises)?
So I try to separate this question in two parts:
1. Crunches reduce the belly?
Not necessarily. Because you just lose fat if you are in a caloric deficit. Therefore, the biggest part to worry about is your diet. Of course activities like crunches or weight training at all (or any other activity) increase your caloric output but the main part to focus on is your caloric input. By the way it is much easier to focus on the input (mostly with tracking your calories or other methods) than increasing your output with exercises like crunches.
2. With more crunches you gain belly size?
No I wouldn't say that. Of course with more strength (e.g. more repetitions or more resistance / weight) you'll get a bigger muscle. But this won't be visible in your abs if your on a natural basis. Maybe you know some pictures of bodybuilders there you can see the increased size of their belly but thats a complete different level so don't worry about it :-)
GREAT question! Actually the answer is more complex then I thought.
Your deep abdominal muscles are primarily stabilizers. These muscles are primarily composed of Type I, oxidative muscle fibers. They're responsible for maintenance of body posture and skeletal support. As such, cellular changes will occur in these fibers. Hypertrophy (or growth) in your deep abdominal musculature will be negligible - as they're frequently activated for sustained periods.
The front "six-pack" muscle (your rectus abdominis) is more of a "hybrid" muscle, having roles in both spinal stability and movement. It was surprising how few studies exist relating to the growth or hypertrophy of this muscle or even it's fiber composition.
From the research I've found:
In males -- yes, to an extent, your abdominal muscle fibers (retcus abdominis) can physically grow (called hypertrophy), however the study did not include female participants.
As testosterone levels are very low in women and approximately 100 times higher in men.
In females I've found no direct evidence supporting abdominal muscle fiber growth (hypertrophy). However, applying physiological and hormonal differences, it's pretty safe to assume abdominal fiber growth would be minimal.
First off, crunches are not necessarily the “go to” exercise for abs because no one exercise is more effective at burning fat than others. And, yes, training a muscle may increase its size. But, in the case of abs, that typically doesn’t happen to the degree that you might think. You must combine proper diet and training for effective fat loss. The goal should be to lose the visceral (or intra-abdominal) fat by combining exercises that burn calories with an effective eating plan.