This is a good point that goes back to my original critique of the current gameplay. I do think land grabbing needs to be limited, but mainly because your ability to do ANYTHING in the game right now is directly proportional to the number of planets you own. You pretty much HAVE to land grab in order to do better than the next guy.
I had proposed research options that would allow you to "do more with less," so you could do something like build a much smaller empire of hyper-productive planets, rather than relying on quantity. The problem is that for this to make sense, research and production/expansion would probably need to be mutually limiting endevours (otherwise you could just expand AND become hyper productive, which would be absurd).
My second critique was that there aren't enough meaningful ways for players to interact beyond fighting over planets.
The way I see the game playing out so far involves a semi-tumultuous early game as players scramble to stake their claims. Once equilibrium is reached in this regard, it's purely a race to see who can research their end game tech first (or establish the most trade routes in that case). Assuming you have reached a reasonable equilibrium in terms of expansion and production, nothing about the mid game requires interacting with other players. Then only one specialty, Warlord, requires any kind of contentious interaction with players to end the game (but Warlord balance is way off so who knows if that is even relevant).
This feels unfortunate to me. I feel like player interaction should be encouraged as much as possible, and not just through combat. I'm kind of intrigued by the whole "holding artifacts" concept as a way to encourage players to fight over something beyond just resources, but I have not yet seen any evidence of that mechanic in the game.
Anyways, a lot interacting mechanics and design ideas at work here, so it's hard to imagine a simple solution, but this is one high level issue that should be considered, n my opinion.