There have always been 3 head sizes, the 5.51mm, .52mm and .53mm and the reason for that are simply different bore diameters and groove depths of various airguns. Over the years I have found out, that PCPs prefer a little larger head sizes on average, either .52mm or .53mm, due to the higher working pressures of the valves in these guns, which leads me to believe, that better grouping with PCP types of airguns and of course, with 5.52mm and higher head dimensions of JSB pellets, has something to do with better air sealing within the barrel, as the pellet flies down the bore. Although, I have noticed on certain airgun channels on YT, that some PCPs, such as the Weihrauchs, don't really like the 5.53mm -on average JSBs; Perhaps those seal the air too tightly and therefore create friction, which in turn causes destabilisation of the pellet?
Anyway, a relative of mine has two PCPs, the Hatsan Flash in .22 and the Benjamin Discovery in .177. He says and I've seen this myself as well, so I can confirm this to be the case, that both of these guns prefer the +.03mm versions of JSBs; his Discovery shoots very well with 4.53mm JSB pellets and his Flash with 5.53mm JSBs as well.
Another thing that I'd like to lay out is, that the JSB company had changed their labelling of the pellet cans/boxes as of 2018/2019, form "///CZ///" to M48/07 or M46/08 on their .22 boxes. The thing is, that they've also changed the pellets when they've done this!
They've changed the hardness/density of the pellets, which leads me to believe, that they've started pouring or mixing antimony into the lead as well and they've also changed the skirts of their pellets, at least of those in .177, which are now a little thicker than they used to be. In a sense and this is by the way my opinion on this move of theirs (of the JSB company of course), that they went to fix/change the perfect pellets and made them a little worse! Kind of what H&N did with its own brand...
My Gamo Big Cat 1000-E now shoots groups with these newly labelled .177 caliber JSBs a little worse than it shot the old ones (the ///CZ/// labelled), which made a roughly half an inch group at 15 yards/ 13 meters and these newer ones now make approximately 0.7 inch/ 18 mm groups. This was also apparent with my great cousins' Cometas, with the almost "ancient" 200 model, that they don't manufacture anymore as well as with the 220 model and with the 400 Fenix model as well. I'm not going to discuss my Cometa model 300, because... cause it's a story for its own topic, maybe even for a whole category of its own! That gun almost made me quit shooting! Maybe more on that some other time...
So yeah, from what I can tell, JSB pellets used to perform better...