The bug

When the horse model was changed to be more in line with the designs of other animals, baby horses were also no longer supposed to grow gradually with age for the same reason. However, this was missed, and it still happens.

How to reproduce

1. Spawn a baby horse
2. Feed it wheat
   →  It gets slightly larger

Cause

Since this behavior is data-driven, it's a really simple fix: replace the scale_by_age component in the baby horse component group with a normal scale component.