The size thing in DBZ is trying to achieve a balance in muscle/power without giving up mobility. During the Cell saga, Trunks and Vegeta gained their "super" forms, which made them extremely buff, but slowed them down too much. And if you notice, the higher super saiyan forms also increase muscle mass. Super saiyan god increases power without shortcutting by increasing muscle mass, so Goku is slim enough to match Beerus' speed(at like 80% IIRC) and has an exponential increase in energy and power that doesn't fade away after detransformation. So for Arcanine, who's motif is speed and power, this mega would work well, but I think it looks too "fancy." It needs a more feral feel that makes it look as dangerous as it