Well, I think Sieges will get a major overhaul soon anyway when Toady gets around to the later parts of the Army Arc. Right now, your suggestion would work ok since attackers just sort of spring from thin air anyway.
But once they're forced to come up with armies out of their existing population, I'd rather see intelligent military strategy. So, yeah, in general it'll be smaller forces against an outpost and major forces against a major city.
But I'd also like to see other factors come into play. What if your outpost is on the only route to a major city? It'd be cool to get advance warning that a major invasion force is going to come through your area. They probably won't bother taking the time to wipe you out, but maybe you'd have to deal with units whose only job was to keep you locked up and unable to help. Or you'd get orders from the king to train up as many dwarves in a short time to send out in hit-and-run raids against their supply lines.
Fun stuff possible, there. Especially if they learn from previous attacks. They get a whole squad slaughtered by a small fortress with lots of traps, they know to start sending in more troops, or maybe even some anti-trap troops or siege engines. No more invincible forts just because they've got 20 stonefall traps along a tight corridor!
But wealth should, in general, have a lot to do with how much attention you receive. A small fort on a major invasion route should get more attention then a small fort out in the middle of nowhere. A wealthy fort in the middle of nowhere should probably get more attention then a wealthy fort deep in the mountains surrounded by outposts. You go for the lowest risk with the most gain. As long as the leader is sane, anyway...