I think it is a fact that Manchester United is a bigger club than Bayern, from the perspective of influence, fanbase and revenue etc. Athletically and competitively they have been much worse than Bayern in the recent years, for sure, but that doesn't take away their status as one of the biggest clubs in the world.
Nothing wrong with what Schweinsteiger said there, especially if he was speaking in the context of popularity in the US and around the world. Even Real Madrid, as big and popular as it is, was dwarfed by Manchester United fans last summer when the two played in the US, I remember.