The bug
When the phantoms are size 0, they deal only 2 damage, when they are size 1, they deals 6 damage.
How to reproduce
Summon a phantom with spawn egg
Let it attacking player
It deals 2 damage.
Summoning a phantom with size 1
/summon phantom ~ ~ ~ {Size:1}
It deals 6 damage.
Comments 3
When the phantom size is 0, it deals only 2 damage in normal mode, when the size is 1, it deals 6 damage in normal mode, same as 1.13 and Bedrock Edition.
Would MC-154372 describe this issue? It seems like the damage the phantom does is based upon it's base damage plus it's size. (6 + [size])
The smallest possible size in Minecraft is 0, with support for sizes ranging from 0 to 127. Slimes also have a minimum size of 0. It wouldn't be logical for phantoms to spawn at size 1. This is a feature request.