FPTP isn’t “also” a problem, it (and more broadly the absence of proportionality in the electoral system) is the empirically verifiable problem. (See, e.g., Lijphardt’s Patterns of Democracy.)

Representation ratio is much discussed, but unlIke FPTP there's not really support in comparative study of democracies for it being a source of adverse results (poor satisfaction, reduce dimensionality of the policy option space, etc.)

Much as the accuracy of sampling is, despite common intuition to the contrary, dependent on the absolute, not relative, size of the sample, representation ratio probably doesn't practically matter much once the absolute size of the representative body gets past a certain point (and beyond a certain absolute size, logistics make size increases adverse independent of ratio.)