I mean, almost complete disappearance of smaller phones is kinda puzzling?

Yeah, my bad there, I understood the previous comment as somewhat of a "Well I don't understand how people can like big phones".

My guess is, as with many other things, corporations pushed towards a certain thing (big phones in this case) enough to make it the default and then the bigger audience just stopped caring as a result.

It is interesting indeed, but I guess that's just it, aside from the obvious fact that the bigger the space, the easier is to design stuff (my previous comment + better heat dissipation + more space for cameras), so probably a mixture of all of these things together and a couple more.