There are many situations where the larger mines become more effective.
Economics:
Using your numbers, it would take only 1 bigmine to kill a battlecruiser, but it would take 12 small mines to do the same job.
This situation is much more common than the escort attacks
Game Limits:
The large mines are essential once you begin to reach the units-in-space limits.
At the extreme case, you will have a choice between 5000 small mines, or 5000 Huge mines.
Deployment:
Another advantage is when laying the mines. A minelayer can deploy X mines per turn. Say a particular minelayer can deploy 12 per turn.
It can then kill 12 battlecruisers per turn using the large mines, or 1 battlecruiser per turn using the small mines.
Sweepers:
The biggest disadvantage of using larger mines comes when minesweepers enter the picture. A MS-V will kill 5 mines, no matter whether they are the expensive large ones, or the cheapo small ones.
Find your balance
EDIT: wow, almost the exact same reply at the same time
[ 20 February 2002: Message edited by: suicide_junkie ]