Normal mergers between factions are standard one planet is kept. Some don't even get planets if they are too far away, like the Fel Empire when it merged with ASA or when the SSS merged with the Black Sun -- neither of these kept territory.
When the GTG merged with CSA? Hypori was the only planet kept -- and then they had to redominion getting every other planet they had lost in that merger.
AE lost the entirety of Chiss space when they merged with LotF. And when Horde merged with CSA, again, they kept all of one planet.
Mandos gained only Dathomir with the merger of the witches. (Granted it was the only planet they had lol )
And back in the day, when The Syndicate merged with OP? They only kept Corellia.
Retaining 1/3 of your faction in a merger is actually very generous. That is only because the Republic has be longstanding since Chaos' creation.
OP was given the same exact option when they were to merge with AE as Coryth restated-- the same one Tef is giving here.
There is no QQ'ing. It just is.
And for transparency's sake, this was what the map would have looked like if OP had merged with the republic back last December. You can clearly see they would have lost all but 1/3 of their territory. ( they would have kept the red part) Same as it is being stated for this potential merger, but instead it is for the Republic.