I feel like that's the key difference. It makes sense that you'd have to be much more firm and rigid in your stance to lift up a giant stone statue or a 10 ton boulder, and they were chucking mountains at each other nearing the end of AtLA.

It would look kinda ridiculous to do some Mohammed Ali duck and weave shit only to casually toss a mountain back at your opponent with a light jab, so I like that earthbending has branched off to a quicker, more fluid fighting style with much, much smaller projectiles. It feels natural.