What does Lan Fan have to be upset about? She seemed to have a modest bust. Though I guess since she can't show it off like Winry and Riza with all that ninja garb might be the reason.

I could be wrong on this but I'm pretty sure the worried one on the left is a grown up May

Yeah. Lan Fan is the one in the back.
If you notice, Winry looks a little larger than normal and is holding her hands over her belly, which would imply she's pregers, which would also imply this is post-story. The one to the right is adult May Chang.

Yeah, and if I'm not mistaken, she is worried about her chest size?