Most simply dont get as fat as you might think --- thy gain water and some fat sure....but they are BIG during contest and sometimes not as BI off-season,,,,just a water/fat trade-off as their composition changes...due to cruising or off periods etc...
They got stretch marks from GROWING muscle throughout years and the skin is an amazing thing as it will succumb to the stress of extra growth placed on it.
Now RAPID -- crazy (all you can eat buffet and hormone buffet) from a newbie user that gains 80 lbs in 4 months WILL get stretch marks,,,, but that was idiotic in the first place right
-FF