MacOS printing and iOS printing are not the same.
Yes, MacOS lets you dictate the page size you want, even to the point of specifying a page size that is, potentially, not valid for your printer.
Here's what I see, for example, from my Mac when I select a page size when printing to my HP OfficeJet:

Note the 'Loaded Papers' option at the top, where the Mac has queried the printer to find out what's loaded.
iOS, for better or worse, tries to streamline this, querying the printer for loaded paper size and offering that as the default, with no way to override it. Why? I don't know. Other than the page size it offers is coming directly from the printer itself (if I change the page size loaded in the printer, both MacOS and iOS update to reflect that).
So I maintain the small page size is coming from the printer itself. Have you tried resetting that? I don't know your printer, but on my OfficeJet the printer control panel asks me to confirm page size any time I load paper in the paper tray.