Not all "full-frame" DSLR's are giants. The D700 and 5D are only a bit bigger than the Rebel, until you add the optional battery pack. I always have mine fitted because of the accessories:

A "full frame" DSLR is expensive therefore bought mostly by professionals. Professional lenses include f2.8 constant zooms which are enormous and heavy. This is a physical necessity - you need a big hole on the front and lots of glass to capture the light.

Big heavy lens means you need a strong metal chassis and lens mount, not a plastic one like the Rebel. Plus I find I need a bigger body just to physically hold onto the thing and keep it steady, especially when I add the cobra flash on the top.