A copy of Super Mario Advance 2 for Game Boy Advance, which is also a re-release of Super Mario World, almost surely uses its own engine and would not be part of the same rom set. Likewise, other Mario games (like Mario 64, Super Mario Bros, etc.) would not be part of the same rom set. So it's nothing about the series using the same engine code or assets.
We're talking bugfixes and different regions for the same game on the same console. But this still has the effect of dropping the size for complete console collections by 50% or more, because most consoles have 2-3 regions per game for most games.
Sometimes, ROM-image-based game titles were based on the same "engine" (i.e. the same core set of assembler source-files with fixed address-space target locations, and so fixed locations in a generated ROM image), but with a few engine modifications, and entirely different assets.
In a sense, this makes these different games effectively into mutual "full conversion ROMhacks" of one-another.
You'll usually find these different game titles compressed together into the same ROMset (with one game title — usually the one with the oldest official release — being considered the prototype for the others, and so naming the ROMset), because they do compress together very well — not near-totally, the way bugfix patches do, but adding only the total amount to the archive size that you'd expect for the additional new assets.
Well-known examples of this are Doki Doki Panic vs. Super Mario Bros 2; Panel de Pon vs. Tetris Attack; Gradius III vs. Parodius; and any game with editions, e.g. Pokemon or Megaman Battle Network.
But there are more "complete" examples as well, where you'd never even suspect the two titles are related, with the games perhaps existing in entirely-different genres. (I don't have a ROMset library on-hand to dig out examples, but if you dig through one, you'll find some amazing examples of engine reuse.)
If you knew where in the ROM image the level data was contained, you could modify it. As long as you didn't violate any constraints, the game would run fine.
You could also potentially influence game behavior as well.
The Game Genie and Gameshark were kind based on this concept. Except, being further along the chain, it could write values coming into and out of memory, so other effects were possible.
So, in the case of Super Mario Bros. ROMHacks, they all use Super Mario Bros. as a base ROM. Then from there, all you need to do is store the diff from the base.
