With a pop cap set to 50 I've seen the 1st wave have 25+ dwarves before. In that fortress I happened to dig into a pocket of olivine so
without any engraving I had a legendary dinning hall before summer even started. I think I got an artifact olivine door to go with it, which didn't help, as I believe the second wave had another 25 to 35 dwarves in it.
In my current fortress, I built above ground, used a dorm for the starting 7 plus first two migrant waves, and didn't break 20 dwarves (required to start getting moods) until the spring of the second year (about the time of the first elven caravan). Since I didn't dig any rock, and didn't bother with exports until the second dwarven caravan, I got very few migrants even during the second year; I think I had a pop of 35 during the second winter. In the third year, however, I dug out a bunch of 3x3 bedrooms that were a quarter marble, a quarter kaolinite, and a quarter malachite. I went from ~35 to 52 in my first wave that third year, and gained another 10 or 15 the second wave, and got to 75 dwarves with the third wave that arrived shortly before the third caravan, the liaison, and the goblin bowmen that killed the liaison. (He really shouldn't have let them guide him to my fortress ... *sigh*).
In short, the pop cab works, but it appears the liaison checks your fortress, goes back to the MH and says, "Oh yeah, they got lots of room!" or "Don't bother, they are full." based on your pop cap and current pop at the time he was there. (this is the first time I've gotten such a clear ignore of the pop cap because the liaison hadn't returned yet.) So, lower the pop cab, and dig/sell less, and you will not receive many migrants. Oh, and get lots of migrants killed by stuff and they won't show either.