From memory, students accomplish this through a combination of three things:
1. Make the belongings smaller
Telescopes are collapsible: they can be shrunk down to fit inside an ordinary suitcase and then extended out to full length when used. According to Pottermore, and I'm sure it's also mentioned in the books, you can also buy Collapsible Cauldrons. So those two items wouldn't take up quite as much space as you think they would. (I can't remember whether there's an actual Shrinking Charm, or anything of the like.)
2. Make the suitcases bigger on the inside
In Deathly Hallows, Hermione uses an Extension Charm to make her purse bigger on the inside, so she can carry a disproportionate amount of stuff inside it. It's never explicitly stated that any of the students do the same thing with their suitcases, but it's possible. Or failing that...
3. Bring more suitcases
Again, I'd love to get a screenshot, but I'm pretty sure the early films show the students at Platform 9 3/4 carrying about half-a-dozen suitcases each. So if they couldn't fit all their stuff in one trunk, and they didn't know the Extension Charm, they could always just buy more trunks.