That's not a trick, at least not one that DS9 invented, though they're as guilty of it as any of the other series. The size of the Bird of Prey has fluctuated wildly since it first appeared, often seeming to appear as a small scout escort vessel and then jumping in size to a full cruiser. The BoP actually started out small and was scaled up as time went on.
It's now assumed that there are multiple classes of Bird of Prey, with the B'rel serving as the small scout/escort and the K'Vort serving as the larger cruiser.
Nice article on Ship Sizing. I did see the difference, like in the TOS movies, where the crew was standing on top of a Bird Prey.
You'd never would know by looking on screen at TNG. TNG seemed to follow the rule that the enemy ship always had to be as large as the Enterprise
DS9 making them smaller was pretty smart, using them as strike and bomb fighters, like the Fed fighters.
It looked cool in Once More unto the Breach.