Not only do I paint purchased miniatures, but I also Paint 3d printed miniatures. I do both "15mm" and "28mm". Not only is there a large variation among manufacturers but also among stl creators. There are many STL creators that "Presupport" there stls, but unfortunately, many support their files for 32mm or 34 mm up to 40mm and everything inbetween, which makes the presupports worthless. Its easy enought to enlarge a miniature STL from 28mm up to 40mm and you can print it, and it will not fail, but if you take a miniature supported at 34 or even 32 and you reduce it, the presupports often fail.
as said above, painting a 18mm is different than the true 15mm, I prefer the 18mm as in 18mm to the eyes of a " 6 foot tall person" and at 18mm I can often paint a black dot for they eyes, but true 15mm, often you cant.
Now 28mm vs say 35mm a lot can be done with the larger version, especially depending on the sculpt. so what gets painted on the show and tell videos? often 35mm. easier to paint great details, it photographs easier but the scale creep is terrible if your actually trying to play a game with them on a table, or even 3d printed dungeon tiles.
I Dnd with 28mm (25m to 32mm, anything larger to just too out of place), I fantasy wargame with 15/18mm and I like the 18mm stuff best.
However, If I could convert everything I own and print too one scale, it would probably be 1:72 which is also considered 25mm and often even called 20mm, though I have never seen a actual 20 mm scaled fantasy miniature, in person or even advertised, most of what (fantasy) players call 20mm is actually 1:72 or 25mm. Wishing is like fishing, we have what we have, and I dont think the scale creep thing will ever be solved.