PC The Bosses are smaller on Celebration Seed, but their hitbox isn't.

[Nakano15]

Steam or GOG

Steam

Single Player/Multiplayer

Single

Operating System

Windows 10

Terraria Version

1.4.2.3

Controls Used

Keyboard/Mouse

I just had that issue while playing. I was playing the Terraria Birthday seed, when Eye of Cthulhu spawned, and his atacks were hitting my char even without the sprite touching my char.
It seems like their Hitbox isn't adjusted based on the size change, and possibly affects other bosses too.

Edit:. Little correction, It seems like the hitbox is actually outside of the boss too. I can hit him from above with arrows, but under It only hits if the arrow enters the sprite of the boss.

 

[Connor Findley]

That happens with me except the hitbox is still smaller and it kind of annoys me. Even the celestial pillars and every boss is smaller than it should be.