Forgot that there was a popcap on species - thanks Martin.
If you removed this popcap and breed huge swathes of dogs, they can kill almost anything. Even an Elite Archer is going to run out of ammo, and then he is eventually going to get tired when he wades in to melee and eaten.
Using Pressure Plates and putting cages around blind corners (for best effect, put PP on the corner and cages before and after), even squads of Archers can be quickly destroyed.
It really needs to be huge numbers, though. I remember once (many fortresses ago) getting 2 elite marksgobbos each leading up a squad of ranged units, and an elite sword or axe-gobbo leading a 3rd squad, with 2 other squads led by elite attacking as well. That sword/axe gobbo killed 5 of my champions when we rushed him and wounded enough others that my melee forces were then far too weak to take on the remaining forces. They also brought a dozen trolls with them. My ranged units might have been able to turn the tide, but the elite marksgobbos had the ability to take them out. The fortress would have fallen if not for a dragon that had arrived on the map just days before who greeted the two squads of ranged gobbos and barbecued them and set the map on fire which like a month later finally caught up with and torched most of the trolls that were busting the place up on the other side of the map. That allowed me to use my remaining forces on the last two squads, getting in just enough damage that the siege broke and they ran off. The dragon fortunately decided to wander into a cage trap I had set to catch mountain goats. Even with a well planned out approach, I doubt 200 war dogs would have done the job.
Part of the problem is that once you unleash them on the first squads, they're now scattered and out of your control. When later squads arrive, particularly if they are ranged, the massing effect from the dogs is lost and there's 300 bolts ready to be unleashed on them as the dogs attack a few at a time.
So you really need to be able to release enough dogs to take care of two squads arriving simultaneously (what I usually see) and then have in reserve enough dogs to take care of the next two squads that arrive. And if you are brave enough to try this with orcs, you have to have enough dogs in reserve to replace those that are lost as you're likely to get sieged every 2 seasons.
So, yeah, you could survive one siege with a smaller population, but if you want to be able to handle recurring threats you need far, far more than you might realize.
(That was an awesome map, too. There was a huge cliff running down the center of the map so you couldn't travel from east to west. The fortress had an entrance to each level which turned out to be a bad layout since it divided my forces, but there was a massive exposed/underground chasm and a pit. There were GCSs everywhere - it was impossible to keep the population down and it was difficult to get a melee army properly trained up because those little bastards would come in and snipe the guys on the ground when they were out protecting the dwarves carrying in looted armor and destroyed trade caravans. Had I only had orcs back then, and known half of what I know now.)