It needn't be that a larger group should always defeat a smaller group within a few days though. Castles when properly provisioned were designed to keep a larger group out for long enough that they had to yield. If there were options like that in the BGS it might help.
However the BGS simply has poor mechanics, the concepts it uses are peculiar and unconvincing. How can a political faction suffer a disease or a famine? Why should it be easier for a small mining community to invade a system of billions than for the far bigger system to invade the smaller one? The BGS would make far more sense if a 'governing' faction was actually in control of a system, so that the domestic factions rivalled each other to be the opposition if there was a civil war, and if foreign factions had to invade with an invasion fleet rather than set up a bulletin board stall like any other faction. Almost every space empires game uses these mechanics.
While true, if a number of players wanted to steamroll a smaller faction they wouldn't have to wait for days or weeks to carefully match influence with the faction (to the point of running missions for the opposition and killing their own faction members, even), check to make sure the faction they're matching with isn't in a war elsewhere,
then declare war, either-- and the fact that all of those are the case means that the smaller faction absolutely has opportunities to block a takeover, if they're clever about it.
Like most games, "realism" isn't (and can rarely be) the goal, just a mostly-internally-consistent set of unrealistic lies, analogies, and symbolism that people can follow along with. "This is not a pipe" and all that.
That said, if both groups have an equal knowledge of those in-game rules, the group with more players (or, when you get down to it, the group that spends the most player-hours on their goal)
should be the one that wins, shouldn't it? You can get into nuances of how to implement the defender's advantage or distractionary, divide-and-conquer tactics but when push comes to shove, if the group that expends more effort is losing an otherwise-equal a head-to-head battle then something is fundamentally wrong with your game's balance,
especially in a game like Elite Dangerous where there's explicitly a limited area to work in. Territorial conflicts are not just inevitable, they're part of the baseline game-- this isn't an MMO where every guild walks into "The X City Guild House" and has their own instanced copy of the interior. For better or for worse, this is what the game is and is likely to continue to be. The saving grace of this is that if (or when) the larger group comes in to steamroll you, it's likely that they'll do it and be done-- most groups aren't going to be long-term interested in "occupying" territory that isn't theirs; they'll just declare victory and go home. And since player factions can never be retreated from their placed systems, you always have the option of coming back in, picking up the pieces, and recruiting in the hopes that you can withstand or even initiate round 2.
but there are. reason for that you can only gain the system controlling station in a system control war imho. if a (small) group owns several assets, it basically works like the line of defense of a castle.
Not quite? I mean, if you lose the initial war you'll still have assets in the system, true, but the first asset lost in a war is the "best" one-- and in that case, it'd be the system control asset. I know Frontier was talking about flipping that awhile ago, but I don't know if it got reverted or just never implemented.