Are we talking big as in "a lot of maps" or big as in the size of the maps?

If it's the former, then a game should only have the bare minimum of maps the game can get away with without being overly extensive. It's always good to have a lot of unique places to go. In many RPGs, you'll see a few cookie-cutter places, but having a snow area, desert area, and a whole bunch of others can make your project outstanding. And they should be detailed enough to the point where you're not just looking at a white screen of snow, or a beige screen of sand, or a green screen of grass. There should be depth and atmosphere to any map, to make it feel like "wow, this is an actual place in a game that looks so lively" rather than it just being "a map".

As for the scale of worlds, that also depends on the plot and the direction on where to take your game. There's even RPGs that take you to different worlds! Heck, I even ended up trying that idea once. But generally, though, if you're going the route of making multiple "worlds", then it stands to reason that they should all have a lot of meaty weight to them, not just being one map you can go to and say "hey, I'm here now, where do I go next?"

If it's the latter, generally, a good guideline for mapping is: "start out small, and then gradually get larger as you map it out". Anyone saying that they made a 300x300 map quickly obviously didn't put in the effort to make said map as lively as it can be. I should know: I made a 80x349 map in RPG Maker and it was HELL to make. It took me almost three weeks to properly assemble the basis for what I wanted in the map, and then another two just to map it out. Granted, this was also a Parallax map which ended up taking a lot longer, but my point still stands. It's also because the general direction of my project is to have those open-ended maps because there are a lot of fights to get into and rewards to claim. But that's another point altogether. A bottom-line basis is you should make your maps large enough to the point where you feel you can adequately give them "liveliness" without going too overboard. If you have to map at 50x50 or 65x65, then so be it. If you have to map at 100x100, then good for you. But there's a difference between mapping and making a place feel alive and real.

That's just my opinion.

Our world was once a huge island.

I can't say my opinion on this matter, as I'm not a good mapper, I just wanted to say that phrase above this one.

To be honest, it all comes done to the game's story. Is it an island, using your example, that sits on top of an active volcano? Is it a floating island? Does the island have an ancient tribe on it or stories to tell? I feel as if that is what determines the final size of the maps.

Now, in terms of the RTP stretches, I do think there's a bit of a problem when your map reaches beyond 100x100, as I have seen some very creative RTP maps at that size. However, at the same time I state this, it still fully depends on the person who makes the maps.

I think, when it comes to overworlds anyway, the best way to do it would probably be a smaller map that actually has some sort of constant scale and keeps track of how time passes. Maybe making the map more abstract and zoomed out so it is actually more map like then a literal representation of the world. Imagine for example a typical tolkien-style fantasy map with a little dot on it running around Indiana Jones style maybe, plotting a course between points of interest, and each journey would take x amount of time.

Then again there is no need to make maps that big either. Not to long ago I had the idea of maybe doing a game that just took place in and around one village and mostly was navigated through menus. I wanted to pack as much detail and make as a realistically sized village as I could with as few maps as possible, but the project sort of fell through Ether way, same basic idea of having a more abstract connection beween scattered maps.