That Girls' Generation fan boy actually saw Sehun, Kai and Baekhyun changing in the washroom... There was a security guard by the door, but the fan boy managed to get into the washroom when the security guard was looking in another direction. In the end, he saw the three members shirtless. (The fan boy didn't go in on purpose, he only went in because he needed to use the washroom.) He said that because Sehun and Kai were dancers, they had some muscles. But Baekhyun was thin, with a ladylike figure.

Credits:
Source: 小智MAX @ Baidu
Translated by: happyluhan @ CODE:EXO
Edited by: star☆light @ CODE:EXO