the Borg's ability to adapt at such an incredible pace to both technology and battle methods is an incredible advantage.
Also, their ships are much bigger and more advanced.
The Borg should win hands down, but the Borg have been written well below their actual abilities since they were first introduced. It's an example of introducing an adversary that's too powerful, so they get taken down more and more over subsequent appearances.
Actually, the borg adapt through 2 methods, sensors and assimilation. The Dominion's 3 species are immune to assimilation and their ships jam sensors constantly.
In terms of size, the speculated Dominion "Planetary Defender" (its existence is unspecified) is 4500 meters in length and a Borg Cube is 3000 meteors in length.
Advanced isn't true, as Dominion phased polaron beams penetrated the shields of all the Alpha Quadrant powers, where as Federation shields held against Borg weapons.
Not only that, but Dominion build ships faster than the Borg, and breed clones faster than the borg can in maturation chambers or assimilate. Then you have to think of the sheer numbers of vessels... Sure a Borg Cube is powerful, but why not just ram it with 10 Attack fighters.... not so tough now huh?