Question about animals.

Ive just hit the 16-year mark on my main playthrough, and I've made a strange discovery. Sometimes the quality of my animal products will actually decrease. I've gone from a dozen cows reliably producing almost entirely large, iridium-star milk, to about two-thirds iridium, a few gold, a silver and sometimes even a plain, small milk. What's up with that?

Ive begun to think this may be attributable to the amount of space i give them outside the barns. Is there a real connection there? Do my digital cows need more room to roam?