Tubes will just stretch to fit the tyre they are in, so too-narrow tubes are almost always usable (though I wouldn't trust an 18-23 tube in a 4" fatbike tyre).
If a tube is too big, it will be difficult to fit without it getting trapped under the tyre bead or creased (which will result in the tyre chafing through, in due course).
The more stretched a tyre is, the quicker it goes flat when punctured, and the more easily it will chafe through on internal toughnesses, such as the edge of a misplaced rim tape.