I got normal sieges in two separate forts without migrants. In first one I had only starting 7 and two kids, I got sieged by necro tower, captured every corpse and necromancer using cage traps. Shortly after I got another siege from goblins, one full squad of goblins (5-8 warriors), siege tag, announcement and overall normal siege behaviour (traps weren't reloaded, so they got in, killed few visitors, one of my guys, than few of them got killed and the rest retreated). In second fortress I have <20 citizens (starting 7, few kids and adventurer with some companions I added to the fort using dfhack). After few years I got my first attack. Siege tag, announcement and two squads of goblins (in total around 11-15 bow/crossbow goblins). At that point 4 of my people were already equipped with copper armor and iron picks, so they murdered invaders in no time. Next year I got another siege, two archer squads again, and in the next few years another siege happened. After your post I began to doubt if they were indeed goblin civilization sieges, because you can also get bandit camp sieges according to wiki. So I checked legends, and dead goblins were part of goblin civ. Also legends viewer shows that The Bejeweled Thief (Goblin Civ) waged The Conflict of Screaming against The Mountainous Wall (Dwarven Civ) in the year 258 (when first attack happened). And this conflict includes three battles: first in 258, second in 259 and third in 262. In total i counted 35 dead goblins so far. I got a fourth siege once, but game crushed, I loaded save before attack and they didn't come yet (year 263 now).
I believe, that in current version population doesn't have any effect on sieges appearance (at least for goblin sieges). And if you check
Wiki:Siege, for previous versions it says about population requirement, but not for current version.