Jarlath said: You know, now I want to find that chapter of the manga but I can't. Just to figure out why he's doing this.
The reasoning is this: Kana is discussing how as he gets older, it's going to get more and more obvious to Haruka how he has no breasts. Chiaki overhears this and offers Makoto this 'special' dress that she wears to make it look like her chest is slightly larger. Makoto is then forced into wearing it, and there's your source of fanart.