There is actually a difference.
The compressor cover limits the size of the wheel that can be used in the R32 turbo.
The RB25 turbo and VG30 turbo have larger compressor covers thus larger wheels can be used that flow more air creating more power.
The R32 and R33 turbine housings are also different in size but do not affect power as much as the compressor cover size difference does as for an example a stage 1 and 2 wheel can be used in all 3 turbine housings- R32, R33, VG30