Possibly it's to do with warm up time if they have to test the car when it's not at full operating temperature. The smaller one will heat up faster.
I've had my eyes opened to this recently, having bought the 2nd user E39 (petrol) which is a straight 6. The exhaust is very odd, there's two 3 into 1 manifolds, each serving 3 of the six cylinders. These feed two separate catalytic converters,n and they're both tiny. Each about the size of a wine bottle. It's only further back that the pipes amalgamate into one fairly large one and go to the two rear boxes which are in series. There is only one tailpipe.
I questioned the 2 cats thing with a specialist and was told that it's done for fast warm up - there's also apparently a couple of secondary air pumps to force air into the exhaust upstream of the cats during warmup. This, too, is simply done for emissions reasons.
But if they test your car when it's just been driven swiftly the 200 centilitre cat will perform equally well. The 200cl should flow better as well.