Its not a surrender... Its the opposite, really. The big empire gifts a planet to the almost-dead empire in order to keep his trading partner alive.
The little guy gets the trade income from the big guy to rebuild a fleet and help out the war effort.
Its like a Terrorist Shell Game.
"This is his Last hideout! We'll bomb it to glass!"
"Oops, he's left and is hiding out in yet another neighbouring country's land"
"Now THIS is his Last hideout! We'll really bomb it to glass and then bomb it some more..."
etc.
Sounds pretty annoying for the person playing the US
But would make an entertaining story.