[PvK]

You're essentially correct, I think. Also, 12 small mines are much harder to sweep than one big mine.

Advantages of large mines are that:

1) You can launch the same number of them as small mines per minelayer component, so you can launch more firepower per turn with large mines.

2) There is a limit on the number of mines that can be dropped in one sector, so with larger mines, the sector can be made more destructive.

3) There is a limit on the number of units that can be deployed in space (highest allowed setting is currently 5000) - so, in the long-term in a large/advanced game, each mine in space reduces the number of mines, satellites, fighters, and drones that can be deployed in space at one time. So, larger mines give more firepower for the amount of this capacity that is used.

PvK