Bought this in a small. Im 5'7 , 139lbs during season. Medium build i guess for background. With a 34" chest.
I considered buying this for time trials. I also own a Assos cS.Uno Skinsuit . The castelli piece is tighter than the assos skin suit for sure. Both hunch you over while standing which is neat. Meant for time trial position not walking around. The castelli piece also has mesh on the back which i noticed is really awesome and wards off thermal stress. I rode the castelli in training on a 58F deg day and was legitimately feeling 'chilly'.
I don't have any data to support how fast this skin suit is, but i FEEL fast while wearing this thing....and feeling fast is part of being fast. I won the first time trial i rode with this. So it worked, duh. Will be keeping this piece for a long time!