A lot of the body build has to do with the genetics of the animal in question. I'll give you some examples from my own collection. All are 100% healthy, have had the exact same care, all feed with insects gutloaded with Pro Gutload. They all come from different breeders. Bare with me here...
Ozzy has been in my care for the past two months. In that time, he consistently eats AT MOST 1 superworm every week or two. He is the fattest gecko I have, and I would venture to say perhaps a little too fat. He looks like a stuffed sausage, to the point he has nearly no neck. Yet, he barely eats. He doesn't gain weight, and he doesn't lose it. He eats as much as he wants, when he wants - and I believe he is genetically predisposed to be his size. This guy is a tank, and could go MONTHS without eating and have little effects (not that I would do that). BTW - he's not a giant.
Lost has been with me for nearly 4 months. When I got her, she was just 6g - and now she's 42g. She eat like it's going out of style - she'll devour up to 20 mealworms in a sitting. She has the strongest feeding responce of all of my gecko - she's even attempted to eat me during feeding time (the rest of the time she's as sweet as can be). She's pretty plump, but I wouldn't say she's overly fat. Because she's still pretty young, she will probably grow much larger over the next few months.
Brimstone is a lanky fellow. He's always been lanky - and he may always be lanky. He's been in my hands for the last 3 months, and has gone from 34g to 48g. Like all the others, he eats as much as he wants, when he wants. He's a picky eater, and will eat a worm here and a worm there - averaging about 10 mealworms a day (spread over the full day). In that time he's grown several inches in length. Somehow, he is about the same length as Lost, weighs 6g more than her, and he is much thinner than she is. The only thing I can see accounting for this is genetics. He's just a skinny guy. I do think once he stops growing in length he will fatten up a bit. I don't think he's too thin, but hes close to the border.
My lengths are as close of an estimate as I can get - ever try to measure a gecko? LOL
My point being - In my opinion the gutload helps them reach their full size potential, but the genetics play a strong role in how plump they become.