it's not the insulin blurring the lines... it's bodyfat and water.
I used copious amounts and when I was big, I wasn't 'crisp' or deeply separated. Then recently (a few months ago) I was the leanest, driest, most separated i'd ever been.
it's also not insulin causing 'growth' of the muscles in the stomach. In my opinion, based on my experience, anyways. It's the ability of insulin to make you hungry alllllll the time, and gh's ability to keep you lean when pounding down food. That much food bloats your stomach. Doesn't matter if you're 12% or 5% bf.
I say 'in my experience', because when I was big (bbing 'big', not when I was pling), my stomach was in the 36-38's. Hard with abs/serratus ect. Now a few months ago I was down to 31 damn near 30, and couldn't push my stomach out if I tried. If it were permanent growth, that wouldn't have happened (esp considering about 18months before that I was at a 41" waist and about 50lbs heavier (heavier than I was then, not now).