In my experience people often do both: Downsize and/or up-size boats. I don't know that there is a generalization.
One thing I can say (which may or may not matter to you) is that in boating costs and sizes/weights of things go up exponentially which is not the case for RV's in the same way. Sure, a Cass A is going to have big, expensive truck tires, but in many other ways a Class B (van based) can be just as expensive or feature rich.
In boating there is a huge difference. 25# anchor vs. 100#, size of windlass, line, fenders, fuel tanks, you name it. Doesn't mean it has to be more difficult (in some ways a larger boat can be much easier to handle) but it almost certainly will be more expensive. And things are bigger/heavier if a system goes out and you have to do it manually.
The marine environment is also harder on things, so things break, corrode, or need "renewing" more often. (On the other hand a good boat can be re-fitted and go on for generations.)
The above may not matter to you; or it may.
Another thing I notice is that although boating is my lifelong favorite, RV-ing is so comparatively easy and stress free. The weather? Oh I take a look, but it's not like it controls my every move. Never once had to sit up all night on "wheel chock watch" :lol: Never had that "Just Let Me Off!"
feeling you can get in a rough or scary situation on a boat.
Okay, sorry, got off on a comparison tangent since you referred to RV-ing.
To get back on track: I think plenty of people downsize just as plenty go up in size. Come here and you tend to find people moving up in size (trawler forum after all); go to a forum for Rosborough, Ranger Tug or similar (trailerable cruisers) and there will be people who have downsized. Both are happy. So it depends.
I don't disagree with "buy your second boat first," but sometimes you just don't know, because before your first boat you haven't had one (or not a trawlerish one). You may not know how you will end up boating, or may "know" you want to boat in one way but it turns out you like something different. Those things may not be known as you shop for your first boat, so you can't buy your second boat first. (If they are, great, buy your second boat right off
)