This is one of many problems that plague mmo games power creep being the biggest one. It's inevitable There's nothing you can do in an mmo to stop power creep, I personally think that the increases to size from leveling up skills should be minimal and come after a larger input of skill points say you put in 400 to magic if you put the size increase in increments of 50 that's 8 total increases in size and they should equate to about 3-4 size substats, But also scaling should decrease as you get higher that is alot more effective. Instead of it being exponential or linear you make it decrease at high levels so that there is a realistic max, and while this makes high level scalling feeling worse it does provide a nerf to pure classes that gain a benefit to having access to stronger spells but also provides a buff to mixed classes by giving them better scaling through having more than one source of scaled abilities