>>4479224
I did some tests on these some months ago as I also did the dumb thing and just bought kits of KF concept and it included a lot of K series stuff.
I also now use X series filters at a minimum and the URTH is my endgame, so I've got a good mix.
Anyway yeah, basically longer focal lengths magnified the imperfections in the glass filter and made it more obvious. I concluded at the time that the K series was passable at 50mm and below but you could still peep the effects. The X series was practically imperceivable in terms of quality reduction EXCEPT at 300-400mm on my crop camera, so again, the reduction is there but only noticable as you go longer. I have since tested my URTH stuff and even at 400mm+1.6x crop there's literally zero noticable quality drop.
Lens sharpness also matters; the K series was fine on my 18-45mm kit lens because it was so undemanding.
As for filter size influencing quality, I'm going to guess you got a lemon or you were mixing series'. They make the B and D series as well but I think they're phasing them out. Otherwise nfi.
Anyway, as with all things buy once cry once.