A MAME romset will contain all Arcade games that have been dumped ( as well as a ton of shit that isn't games and you don't really want) regardless of whether or not MAME can currently actually run them yet, or not, they are their in the hope that they become playable with the MAME emulator in the future. in total out of a full romset only around 2900 are considered working games.
Doesn't matter as far as Launchbox is concerned, however if you want to break a romset down and just pull out the games you want, then you MUST use a Non-Merged set, pulling out, or deleting games from the other sets will break completely unrelated games due to the way they are packaged, which is why Non-Merged is by far the biggest pack, each individual game zip has all needed files in it, hence the size increase as something like say the NeoGeo.zip which is a needed bios file, is now in that pack several hundred times, whereas it is only in a split, or merged set once.