(Also reported for Bedrock Edition at MCPE-153770.)
Explosion damage scales based on how close the damaged entity is to center of the explosion. However, the position of the entity is handled naively, with the result that higher damage is done by explosions centered closer to the entity's feet.

Steps to reproduce

1. Turn off mob griefing.
2. Place a solid block above your head, so that you are standing in a 2-block-high gap between 2 sold blocks.
3. Summon a fireball at ~~~, then look down and punch it.
4. Summon a fireball at ~ ~1.68 ~, then look up and punch it.

Expected result

The fireball summoned at ~ ~1.68 ~ does more damage because it explodes closer to the center of the body and/or the head.

Observed result

The fireball summoned at ~ ~ ~ does much more damage, almost 5x.