[JMHawkins]

Large ships will go to a major base at the end of the turn if they can reach it (see note below). But if they've been damaged badly enough, they may be foreced to a smaller base nearby the battle. In that case, they may remain in port at the minor base (in fact, may be required to if they are really in bad shape) to conduct repairs. Minor bases can repair large ships under these circumstances, but it will be a slow process, so as soon as the ship is patched up enough to reach a larger base, you'll want to move it.

For ship size rules, the four "home" ports (Brisbane, Ceylon, West Coast and Home Islands) are always considered Major ports, regardless of their damage level. TFs sailing from one of these ports will still suffer combat efficiency penalties if the port is badly damaged however.