I expect that, as is the case with many video games today, Pokemno will eventually allow you to customize your character's appearance. Want to be fat? Go crazy. Want to be rail thin? By all means.
The trailer showed the MC looking in a mirror, why would a mirror be there unless you actually could change your appearance? I mean just to show your reflection would be kind of pointless...
That doesn't follow as well as you think it does.
The mirror could easily just be an aesthetic choice to showcase the change in perspective / graphics, or you could be right, who knows.
I doubt they'd let you change your proportions though, kind of like in FF 13-2. There was a mirror there, but you could only ever change your costumes.
In any case, until we are presented with protagonist customization, devs are gonna go with the standard for an analogue. In order to best appeal to the masses, the smartest choice is to design the character as average, right in the middle of the weight scale, and right in the middle of the height scale, especially for a franchise as global as Pokemon.
We don't have customization yet, because Pokemon as a franchise is still going through major aesthetic changes. I can see it happening in the near future, if not in these games though. --- The Official Luigi of the Super Smash Bros. Wii U Boards