IMHO, it may just be the lighting/editing in those promo-style shots. They have that "glamour shots" style which tends to make things look very smooth.
[Link to Huge Pic] <-- They look pretty muscular to me. I'm not seeing any "skinniness" there (aside from the females in the pic, those elbows look dangerous lol).
Honestly, I think they look very formidable, not to mention huge!