It is based on population. It is a 'hidden' stat but every asset has a population figure. This determines mva.
with the caveat that the controlling station is always space side not planetside.
some weird systems exist where the controlling station is an outpost rather than one of the larger station types - but you don't know until after the conflict. We have had to repeat the conflicts to secure additional assets.
And yes, keeping your faction alive in a system is almost as good a story as taking a system or expanding. Just as death is part of life, the BGS is not always a smooth run of success. In our case the battle was long but finite due to the skimmer mission limitations introduced. That said we had also put in place BGS measures to mitigate the skimmer effects through limiting the availability/effects of the skimmer missions. It was an initially extremely frustrating experience that we turned into an interesting challenge and it added a new chapter to the story of AEDC.
I am still somewhat on the fence about whether such adaptation is entirely possible in 2.3 in high BH areas - that is probably not going to go away without a mechanic chance. And my sympathies go out to anyone who has had the misfortune of being located there (as I do with anyone close to a rare good or engineer or anywhere that attracts prolonged traffic and activity).