I think having 5 full sized people attacking a single person would look really silly in a battle. In order to fit everyone on the screen the would have to essentially zoom the camera out to the point where the party would still look the same size on the screen, only with a far less imposing boss model.

At its heart the FFIV battle engine is always 'my side vs your side' and if you have 5 people on one side and one on the other, the screen will look very empty and awkward on one side. And the character sprites won't be any bigger.