These aliens, too, live in eusocial colonies. But the males have shrunk and atrophied to the point where they're the size of mites. And the queen has grown. The queen is the hive. A fully mature queen is easily a kilometre across, a single sessile organism that houses and protects the workers.
And let's make this a tool-using sentient species: the queens are long-lived and very smart. The workers are the size and intelligence of dogs. Capable of limited forays outside the mother-hive, but not great at independent thought. So what happens when these creatures want to go into space? For a quick orbital flight, you can stuff a few workers into a partly automated capsule. But for anything more involved, you either need fully automated probes - or you need to launch an entire queen-hive into space!
Now the queens don't start out that huge, of course. Freshly hatched, they are the size of a puppy and capable of movement. After all, a new queen needs to move away from her mother before she can settle down and start becoming a hive. But at that point, the queen is just a kid. By the time she's the equivalent of a human teenager, she already weighs thousands of tons. So it's a difficult trade-off: finding a young, bright queen, educating her, uprooting her and fitting her into a giant spaceship. And after a few years, she will need to land and settle down again, or risk permanently stunting her development. So most of their spacecraft are small, automated probes, with a few huge craft crewed by a single "teenage" queen-hives attended by hundreds of workers. Eventually, their technology advances far enough that with a massive engineering effort, they are able to boost a mature queen into orbit, encased in a multi-kilometre-sized ship.
At long last, they have an actual adult out there to watch over things.