Voyager was "theoretically" a weaker, but much faster ship than Enterprise D. However, according to the Voyager episodes, the ship was dozens of times more powerful than than Enterprise D.
Look at the fact that a lone borg cube wiped out something like half the federation ships. in Best Of Both Worlds. And Enterprise D was powerless to stop it.
Now Voyager takes on a Borg Tactical cube, you know, a borg cube amped up to the inth degree. And single handedly cripples the cube. Wow, either Voyager is tough, or Brannon Braga needs to be taken out to the wood shed.