Clearly the spell which inflates Aunt Marge in book three isn't a standard spoken spell, and is cast wandless. So there is no real 'spell' to speak of. But what is it most like?
Clearly she expands a considerable amount so it may be similar to engorgio, however she also floats far more easily than she should be able to by sheer buoyancy (She'd need to have inflated to about ~80 cubic meters to get any sort of lift, which is roughly 9 foot in diameter!) so there is some sort of lift element to this charm.
What is the closest spell to the effect she feels when she is inflated?