The upper body shape can be explained by her wearing armour; so it's the same reason why her ankles and feet aren't shaped like actual ankles and feet. The rest of her body is very slender and feminine; the visible part of the face too. Including in this image - you've just watched too much bishounen anime if you think this face looks male.
Need I point out that it's repeatedly stated that it's Zelda in disguise, having been trained by Impa. Ruto refers to her as a guy because, hey, that's the point of the disguise. But it's not meant to fool the player, it's meant to fool in game characters. Maybe she changes her eye, hair and skin tone but she can't change herself that much magically because she still needs to wear a mask.
Sheik even has a girl's voice. Link always sounds male, whether it be boy or man, but when you hear Sheik's voice it's a girl's voice. SSB also just says Sheik's female; yes it's a different game but it's a game that takes its characters from other franchises and doesn't change anyone's gender or anything. We don't get a male Samus just because she looks male in her suit. It's just that everyone knew Sheik's gender by that point so it was no longer a spoiler.
Crisisaddict said: Cough, Sheik is a woman, the Super Smash Bros. Sheik is a man, and Super Smash Bros. is technically canon. This is FACT. So in general every time sheik is in a pic where it isn't obvious that she is a man, It will more than likely be tagged reverse_trap.
The SSB Sheik is ALSO female. Read the trophy descriptions!