It's probably a scavenger. The fact that "regular" wolves get no where near that size is a hint that the costs to being that size -- lessened agility, higher caloric requirements, etcetera -- are not "worth it" in terms of evolutionary selection. For that reason, this larger wolf is probably not a pack hunter like a "regular wolf."
I'm not up to date on this issue, but I remember a theory going around that the tyrannosaurus must have been a scavenger for much the same reason that your wolf would be. It's size would have made hiding/sneaking more difficult and fast chases downright dangerous (due to risk of injury). On the other hand, the size does make it scary and likely, then, able to scare smaller predators off kills that they made.
If you don't like the scavenger angle, I see one other obvious one. Big cats get much closer to horse-size than wolves. I think tigers are the biggest, but lions are close and socialize much more like wolves. I don't have the expertise to say why they ended up being so much bigger. My guesses range from bigger prey to different (likely more effective) hunting strategies to competition with other large predators/prey that created an evolutionary arms race (e.g. for lions: hippos, water buffalo, rhinos, etc. are a lot scarier than deer
-- for tigers: they've been known to kill grown crocodiles, water buffalo also, etc.) to intra-species competition. If you feel like you find a satisfying explanation for the wolf vs. big cat size discrepancy then you may be able to adapt it to your purposes.
Personally, I think the scavenger angle is much more plausible. Dogs and cats have significantly different builds and I don't imagine the wolf building scaling as well as the cat's, much less scaling to be 2-3 times the size of
the largest cat.
It occurred to me shortly after writing this that it's also plausible that the wolves were artificially selected-for/bred for size. Medieval knights did this with horses, so there is a direct historical analogue. When I came back here to check whether this explanation fit into the parameters of your question I noticed that a commenter made a similar note about breeding. I decided to add this to my answer anyway because it may be important to you that war horses, not just draft horses, were also bred selectively since that is a better match for your setting.
Personally, I still like the scavenger angle. I would use that or maybe have them be large scavengers that have been bred to be even larger by orcs. It feels really unlikely that a "regular" wolf could be bred up to 10-20x it's size. On the other hand, if your scavengers were already 5x the size of a "regular" wolf then why not try and make them a little bigger still? I think those two ideas could complement each other.