Most variety of diameters only in .22 - why is that? And why undersized?

You are spot on! Seems like .22 barrel manufacturers have more different bore sizes than in .25 for some odd reason. My RAW's LW non choked poly barrel only shoots .2175 diameter NSA slugs accurately and my Taipan V2's CZUB barrel prefers.218 NSA slugs! My friends M3 barrels likes .2165...but most .25 have only a couple sizes available that shoot accurately from most barrels in .25 it seems.
 
Chokes and smaller diameters to resize bring their own issues where there's less issue in 25. Caliber restrictions in countries like south Africa where the biggest salesmen connected to manufacturers live.... Maybe. 22 has been mainstream longer than 25 in the modern market and maybe was less standardized but maintains a higher demand. I'm only guessing here.