well...is the disc exactly in the centre between the 2 sides of the caliper? even 1/2mm can make a big difference, so most likely either the wrong offset disc or inaccurate caliper brackets. The good news is, inside too tight can be corrected using shims or washers between the hub and the caliper to space the caliper inwards a little. The G4/D2/Ksport 8 piston brake kits come with shims for this exact purpose because they know not to trust the factory's "tolerances"
2nd....some pads are simply bigger than others new. You can always grid a little off.
3rd....if it is actually a piston not retracting ie if you pull the caliper off and you can't get the piston to sit flush....then you have a seized piston and this will stuff your braking performance. In this case remove the caliper and get it rebuilt, may be expensive if the brake piston or bore are damaged.