Wingnutx and Geo both used the B button like DoubleO7 said to make their text bigger. But the default size is what you see in the other posts and that is determined by your browser. I don't know what browser you are using but the defaults of your browser are what determine text size. I pull down view and use the "Zoom In" a couple of times and it stays that way for that tab. If you want to always get an increased size, it requires bumping the zoom in the browsers preferences/options.
As far as changing the font size, I do think that is possible, but on all my browsers, it's normal size already (same size as classic though different font, and larger than modern view was). I think it's situational from the beholder's perspective so what I see isn't what you see. I'm comparing it with other sites I have open, and Sony is the same size, the Kroger store I use is the about the same, to software sites are slightly larger (both blog sites). Other factors make a difference like a person's monitor resolution. Be interesting to see what everyone sees but I'm not sure how you'd do that. I have to wear reading glasses to see any site, but this is pretty much the same as what I see on others. If you are thinking phone, there is a chance Kim had the phone interface to classic bumped up a little. I don't know, I can't use a phone for web browsing. All sites are far too small because the screen is too small and it's not efficient to zoom situationally as you mentioned, I'd stop using the net if all I had was my phone. But yes, I think a global font percentage increase is possible. We haven't got far enough to have that sort of thing ready, but we can make a dark version, a large version etc (dark meaning dark background with light letters which helps with some vision impairments, bumped up fonts would be kind of like that).
The Robinhood post you see has two things there. One has a quote and nothing else. I'm not sure why that would even be posted but it's fonts are definitely supposed to be fine print, and most of that type of quote can and should be ignored when viewing a post. It looks like a mistake in posting (like pressing reply then post reply). Quotes irritate a lot of YT people which is why I created the "Hide Quote" preference, but depending on how someone forms a post, a quote may be necessary, so I just get rid of the opening quote that appears to reply to someone. They are there because they have to be, but they are hidden. The kind that can't be hidden are like this:
Alexander Hamilton said: Something, something and something.
You can't hide that kind or the post loses meaning. Those fonts are supposed to be tiny though.
The changeover wasn't forced by management. The problems with the old software were far more significant than I've posted. Security was the biggest and it's not good to talk about that sort of thing online. It was an accident waiting to happen and the hackers are now government sponsored in several countries and top notch. You have to keep current with OS, computer languages and other things or the server becomes a dangerous thing to leave on a US net (just as PCs are if they are out of date, but the ramifications are larger with servers). The old software broke with every attempt to keep security current, and one-off bandaids to try and around that are dangerous too. A person like myself can't possibly be sharp enough (especially at my age) to ensure safety.
Our problems weren't as important, but they were nightmares for us and angered a lot of the community. Broken threads, gibberish throughout the posts from modern character sets and phone, broken features because of a complete change in how database software works now. It's all adds up to lack of being able to support the system. We were actually very fortunate the last few years to never have had a true crash. If we had, I believe there would have been no recovery. All software would have required a basic rewrite, and if I rewrote it, it would be a week before new bugs would start showing though.
I was planning to write this stuff up in detail because so many people have the misconception that we were interested in doing gee-whiz modern stuff. That ain't us.
PS I found the emojis even though they were hidden. 
