Its like luffy become big baloon. And getting hit/push by big baloon isn't doing much damage at all.
Okay, I understand it's easy to miss this if maybe you're just reading scans or something but it's not a balloon.
Luffy bites into his wrist because he literally blows up his bone. Hence 'Bone Balloon'.
Quite different from his balloon technique.
So the attack isn't actually just air, he's literally expanding his bones. Maybe it doesn't make perfect sense in real life, but it makes perfect sense in cartoon physics.
And believe me, a wad of calcium that large slamming into you would hurt like a mutha.