You missed a zero with the 1921 figure, but it is an interesting question to consider: what exactly is the optimal size of a national assembly? Does it scale best to the square root or cube root of population? Or some other exponent? What are the constant factors? Do larger multi-regional blocs like the US and EU require different styles of representation, and if so does that change the scaling factors? The US could probably do with at least 600 though, minimum.