This is a really petty thing.
I noticed they have different sized waists


Note the vines do have the same size, but the golems from the mod have their waists scaled down a little bit.
The upper pixel in the vanilla golem's waist is half inside its torso, while the mod's golems' aren't.
It's more clear when seeing inside


As well as when seeing it from the side
The vanilla golem's waist bulges over its legs


I couldn't help but notice they looked slimmer than the Iron Golems

I currently use a custom model for the golems. Matching the Iron Golems 1:1 is not a priority, but thanks for pointing it out so I could think it over :)