When a baby cow grows to an adult, the hitbox of the cow does not get updated to the adult cow's hitbox, and the full grown cow can still travel through 1 block high spaces.
Upon relogging, ALL cows (baby or adult) are given the adult cow's hitbox size. (I.E. babies can no longer fit into 1-high areas)
These hitbox glitches do not apply to damage taken by the cows. A "short" adult DOES take suffocation damage if its head is in a solid block, and a "tall" baby DOES NOT take damage if placed in the same 1-high box.
These hitbox glitches are not displayed in the F3+B mode. (I.E. "tall" babies still have their tiny white box, and "short" adults still show their larger white box)