Not sure if this is a satisfying answer, but there's no single reason for this. Mostly it's owed to the Anime influence stylewise, where the slim type is more common, but it's also because of there already being quite a number of bara style games out there (almost exclusively as far as furry themed visual novel type games are concerned). So in a way I'm also aiming to fill the niche on the other end of the scale. Lastly, to some degree it's simply personal stylistic preference.
In any way, I hope the characters are going to have enough redeeming qualities of their own besides their build to make up for it, especially since the main focus of the final game will lie more on the overall story than on the romancing part. ;3