Why do people think a bigger AoE equals a stronger attack?

[Punk Hazard]

AOE =/= Damage

Madara detonated a Hirudora directly in front of Gai's face. The explosion was huge, yet Gai only damaged a few ribs and was still going.
Naruto's Lava Rasenshuriken Bursted to the point it bisected the Shinjuu, yet Juudara was not fully sliced.

That has to do with individual durability.