hmm, it's not the polycarbonate but one of your slicer settings that's at fault.

I haven't done much with pc - but the bits i did print (and yes it's polymakers) were dimensionally accurate.

So, somehow you've changed the extrusion settings. Either the pc profile is set to use 3mm filament (cura's default) or the extrusion multiplier is way over 100%

Although if the filament diameter were set to 3mm and you were using 1,75, it would have the oppositre effect - so forget that.

What printer are you using ?