The reality is that the breaker size is determined by the wire size. Too big a breaker and the wire becomes the weak link in the system and will burn in an overload situation. The breaker size (ampeage) is determined by the wire size (gauge) which is determined by the sum of the load and a safety factor (amps). Failure to do this properly equals fire.
mj, you are correct that wire is sized depending on the load and a safety factor. However, the breaker/fuse is sized either to the wire or to the load, depending on the location on the electrical system. A very simple system will have a 12V battery, a DC Main Panel and branch circuits.
The 12V battery has a fuse protecting it, and that one is sized to the battery wire. The Main Breaker of the DC Panel is sized to the total load on the panel (sum of all circuit currents). However, on the individual branch circuits is where things change.
One of the branch circuits has a Rule 360 bilge pump, which draws 2.1 amps and will probably be wired with a #18 gauge (not taking into account voltage drop, wire length, bundling or any other de-rating issue). Outside of engine space, a #18 will give you 20A according to industry tables. The fuse or the breaker is a 2.5 Amp. The reason behind it being fused to the load on the branch circuit is that if you have a short circuit of say 18 Amps, and your breaker/fuse was 20Amp, the breaker will not protect the load. However, if you fuse to the load, with a 2.5Amp fuse, you will be protecting BOTH the load and the wire.
So, in branch circuits, you size the fuse/breaker to the load.