Why the fuck are Venom's legs so small? Do they not know who the fuck Eddie Brock is? Eddie is 6'3, 230 pounds. Before he even obtained the Venom symbiote, he was at the peak of human strength. So why does he have bitch made quads? Don't even get me started on his calfs.
His legs aren't small, his upper body is big. Also Venom's body mass and shape is based on the make up of the symbiote, not Eddie himself being buff.
Uh...what? While the Venom symbiote does have the ability to increase it's mass on its users, the overall functionality of the alien, especially in the beginning of it's run, has been nothing more than a suit. Yes, we've seen the symbiote increase it's proportions on Mac Gargan or notably any version of the Ultimate Venom. But that was mostly tied to writers displaying the relationship between the symbiote's size and it's emotion or it's user's emotion. But during Eddie Brock's run from his very first appearance? It's been a suit with no notable mass enhancement.
So yes. Venom's appearance on Eddie Brock is largely due in part to Eddie Brock's aptitude to lifting weights. Same reason why Peter Parker's Venom was smaller due to his overall frame being smaller.