there is also the i.reddit.com prefix. Sometimes the old.reddit.com interface can make titles very large, and body text (and links) very tiny, on mobile. I can only assume this was intentional, as it happened when the new UI was introduced.
This started happening in the past month for me with Firefox on Android. It turned out that it was due to some sort of Accessibility / font resizing setting - disabling that setting caused it to use the correct font sizes again (and apparently, has a large performance and battery life benefit as well).
While this is quite interesting, I've moved on these days. It's clear that my manner of use is not desired, so I'd rather not set myself up for disappointment when I arrive at the inevitable outcome.
I do appreciate you commenting though, because I sure wondered what was going on!