@Dhranios
Those saying it's not a bug but a feature request, it was already requested.
https://feedback.minecraft.net/hc/en-us/community/posts/360074839111--size-command
Meaning that if you implement it in any way even if it's just attributes, it should effect all mobs, including entities.
@Jiingy it works on armorstands. Also notice I updated it doesn't work on the enderdragon.
Either way, it should fix one or the other or both
Will try in superflat world tomorrow as neither of those happened for me.
not sure what happened but 18w19b fixed 18w19a's problem so now this can be closed.
@Greymagic read my comment