Romulans are necessarily weak(in numbers not technology) given the apparent small size of the Romulan Empire.
North Korea is puny. Yet it offers a deadly threat far exceeding that of larger and better equipped powers because it is extremely proximal to the most vulnerable parts of South Korea, and has made the necessary preparations for an attack that calls for no further buildup, troop movements or other warning signs.
Similarly, the Romulan Star Empire might be dangerous exactly because it is small, and therefore has its hard core of power bordering right on the Federation, rather than somewhere in the distant depths of a vast reign. And because it's an ancient but unyielding foe, this hard core happens to rest against what used to be the outer border of a very small initial Federation - that is, right against the UFP equivalent of Seoul.
The female changeling thought the shipyards of a bottled up Cardassia Prime would bleed the Federation dry?
She appeared to be correct. The Dominion beachhead force had suffered strategic defeat after strategic defeat when it came to ship resources: yet another major shipyards destroyed basically every season, key reinforcements lost in the wormhole, further reinforcements cut off for good. And the force had started the fight with a fairly limited number of ships, a number that Starfleet had been able to count pretty exactly when it moved past them from the wormhole to Cardassia - and this initial force had been far from overwhelming, as we necessarily saw a lion's share of it deployed against DS9 in "Call to Arms" and it didn't amount to much. Despite all this, Dominion numerical superiority was the one quoted thing threatening to bring the Alpha Axis to its knees. Clearly, the beachhead force had truly astounding shipbuilding capabilities, amounting to a deadly superweapon comparable to the A-bomb in WWII (perhaps in the shipbuilding's favor).