That was my first thought, but all the pistons seem very free when pushed back in (they are Brembo 4 pots).

Not sure about the piston sizes though, will have a look (passenger side wear seems pretty even). Funnily enough the wear on the Saff pads looks slightly similar on the drivers side. Could it be anything to do with the way a circuit runs?!? i.e. Clockwise?? Long shot, but other than that i'm clueless!