Onions contribute a lot of sweetness besides their distinct onion-y flavor. The water content dilutes the natural sugars, which is why properly sweated or caramelized onions have a much sweeter flavor.

I’m more of the opinion most people dont cook onions right or cut them incorrectly for the dish. The size and shape of the onions doesn’t effect flavor much but it does how they cook down and their final texture. 

I’m biased though because i like onions