I have wondered about this, especially on milking breeds. My opinion, based on all the old pictures I've looked at, is that considerable improvement has been made thru selection over the past 100 years.
If we judge them by their contemporaries, I'd make the observation that the farther north you go into Scotland, the bigger the testicles. No explanation necessary.
1. 'Welsh Bull - "Wern Goalkeeper" first at R.A.S.E. show, 1910'
2. 'Galloway Bull - "Excelsior" winner of first prize, H. & A. S. shows, 1901,1902 and 1903'
3. 'Aberdeen-Angus Bull-"Maramere" (18160), Breed Champion, R.A.S.E. And H. & A.S. shows, 1903;