Pulley ratio would also come into play here.

I don't know for sure, but I'd be willing to guess the E320 spins the compressor slower than the E300 at idle engine revs and needs more displacement to compensate. The E320 would have a higher red line than the E300 so the 320 must make sure the compressor isn't over speed.

You may be able to find dimensions on the Denso site. Unless the AC is deficient in capacity at idle due to low freon flow, I'd tend to install the recommended compressor.

If your high side pressures are high and low side low at idle, a larger compressor won't help since yo are already moving as much volume at the condenser / expansion valve can handle.

If the high side is low and low side high, more compressor volume would help as long as the pressures come in line if the engine is run to say 1500 RPM.