It's the contrasting neck for us. 

 Our signature 2-ply mesh neck "pair with everything" super-stretchy bodysuit is back. It's a must-have for the Holiday Party Season

Poly/Spandex Blend

Stretches nicely all over, especially in the crotch.

The model is wearing a small. 

Small fits up to 5/6
Medium 7/8-9/10
Large 11/12-13/14