Moons come in a lot of sizes, by our own galaxies evidence it would seem. I think the moons of some planets are larger than Earth, are they not? I'm not an astronomer so I'm not entirely sure of that, but I think it's true.
Like they said in ANH, "He's headed for tha small moon", so they just assumed it was a small moon of Alderaan I guess.
Though you'd think Han knew something about Alderaan, I dunno, like that it actually had a moon or two floating around, and where they would be. Maybe he's not Alderaan expert though I guess.
Anyway, I think it's simply a matter of moons not having a set size, and that they can be bigger, or maybe even smaller than either Death Star.
Yavin IV is a moon as well, and it was substantially larger than the DS I also.