Actually, I think the font switches back to the default size when looking at the comments of posts from instances other than lemmy.world (my account is from .world).
IMO this is a bug with Chromium based browsers on Android. I have the exact same issue, not just on wefwef but on every other site. Some text elements are of the right size, some small, sometimes they flip-flop
Try Firefox for Android. AFAIHS the text size is consistent here
Well, about that, I actually use firefox as my main browser. I only installed wefwef through a chromium browser because I couldn't figure out how to install it through firefox. When I go to vger.app on firefox, it pretends I already have it installed and the 'install' button on the three dot menu does literally nothing. It's true that chromium screws font size on every site.