Hi!
No, the diameter must be the same, if you mean the bore (75 mm). The distance between the pin center and the highest point of the piston crown may differ. The volume of the "intruder" as well. Or I could ask in other way: is the higher compression ratio in eg. AX GTI is higher than in AX SPORT because of the piston? (9,9:1 vs. 9,6:1) . I just don't have ax gti or 106 xsi homologation...