Pixiv user-added tags include "Reality is a harsh mistress", "The culprit are the Devs", "73" and "Stand User".

Edit: Pixiv comments also mention that most of the other uma-girls also had flat decreases across the board, with Oguri losing 4cm.

Edit2: There are also these gems:

Верный said:

Hold your horses! Ever heard of Schrödinger's Boobies?

This is a concept from Quantum Mechanics, where boobies exist in a superposition of various states until you turn 'em over and look. In other words the possibility exists that they can be HUEG badonkadonks.

That's why we should turn 'em over and l—

青空システム said:

—WAIT! It is also written in the Kojiki, the Records of Ancient Matters! Ninja Slayer reference
"Flat is a status symbol! A rare commodity!"
Which means... a flat Cafe is the bes— surghhhh?!! [/stomach kicked]

Oh wow, I never knew that the devs also nerfed the girls' bust sizes to make their designs less sexy to please the IRL horse owners lol.

Context: The Umamusume designs have to be vetted by the IRL horse owners, and initially a good amount of the designs were very sexy and so the IRL horse owners weren't pleased at all, and some even withdrew their permissions (so that's why we're not gonna get King Kamehameha even though it had an Umamusume design made from the start). So the devs had to change the Umamusume designs to make them less sexy.

You can see it as well in some Umamusumes now with their initial race clothes design being scrapped and a new design being made for the game, like with Hishi Amazon or Super Creek.

On a related note, the English wiki still has Cafe's listed as 81cm, but the major Japanese ones have it listed as 73cm.

Edit: Oh and funnily enough, when this post was upped, Manhattan Cafe had exactly 81 posts.

Edit2: Oh, Hishi Akebono had hers increased from 94cm to 99cm.

Even with that "update" fanarts will not go to the whims of statistics.

Just like when they change Nishi Amazon's.