let me ask you a question.. (but disclaimer, all my experiences are with the Alterians, with bonuses to economy, luck, and a think a few points of research and diplomacy)
are you researching any diplomacy techs early outside of universal translator? Are you building influence buildings? Even when terrans have claimed a few planets in my space, and in the middle of my space for example. they barely get any push outside of the single system they have taken. I
If they are claiming a planet after i have been there a while, often they get no radius claim at all.
Building special high influence buildings on your home world, then i build influence buildings on every planet that has a bonus influence tile. Lastly i build influence buildings on planets on borders, and/Or build influence space stations in areas i want to push boarders, or keep things like asteroids, or mines in my space.
You did not mention how many AI's you are playing with, or what size map, or numbers of planets.. But for me i have found larger maps, with less stars and fewer habitable planets, has things spaced out a bit more, giving more meaning to the idea of territory.
Another thing you can do is choose 3-5 races on a larger map, and just not pick terrans till you get the hang of things a bit more if the terrans bonuses to diplomacy has been an issue.
another bonus to getting more influence outside of pushing boarders, you can use small amounts of influence can be added to trades to get techs, which really helps. With more influence buildings you build this faster so you regain what you loose in trades faster which keeps you from going into influence dept, and loosing ground in claimed territory. I often find i can trade a couple of cheep techs along with 100-300 influence points to make great early game trades especially with minor races. though this does work well with friendly races you are close to as well, to help form a bond.
I hope some of this helps
