Eh I don't know if that would be OP. It should make for an interesting discussion overall. The Hive World obviously have populations of billions and corresponding industry and the like to draw off of, and two thousand or so Space Marines is a lot. And a few thousand Imperial Guardsmen that can be deployed (I'm assuming the Ultramar worlds obviously have their own PDF's and defense fleets etc) but I wouldn't say the Imperial Guardsmen are somehow clearly superior to Mass Effect ground troops.
Battlefleet Gothic has an Imperial Battlefleet consist of about 50-75 warships in total which in all honestly, seems more threatening to me then the ground forces. And that's mainly due to the Citadel Races having a treaty mandated number of dreadnoughts. I think by 2186 according to the Mass Effect Codex there were 86 Dreadnoughts in the Citadel Races (39 Turian, 20 Asari, 16 Salarian, 9 Human, 1 Volus). But Mass Effect has a lot of smaller vessels.
The largest single race fleets were probably the Geth and the Quarian Migrant Fleet, with the latter having some fifty thousand ships of all types. But that's not all Navy ships obviously. As of the First Contact War, the Systems Alliance had 200 warships which was only a few decades ago and they built up their navy massively since then including the Dreadnoughts. Other insights are that at a minimum, about 28 cruisers were lost at the end of Mass Effect 1 and the Battle of the Citadel and the Systems Alliance typically has 4-6 frigates per cruiser. But without knowing any sort of ratio of dreadnoughts to cruisers, we can't really extrapolate anything.
Regardless though, the Citadel races having "hundreds" of ships seems likely. If we just assume 4-6 cruisers per dreadnought that'd be 430 Cruisers which are actually pretty strong combatants in the Mass Effect verse as well. Then 4-6 frigates per cruiser and that's about 2200 Frigates. Add the same amount of corvettes (typically frigates and corvettes have crews of around only a couple dozen) and you'd get like five thousand ships as one very low confidence estimation of the Citadel Flee since we aren't even discussing the Hanar, Volus (for the most part) and Elcor etc. Then you can add the Geth and Batarians and various Mercenaries and Pirates and Terminus fleets and the like as well.
The Ultramar Battlefleet would obviously be stronger than any single fleet in the setting I reckon of course. Even if you just use half of it for deployments abroad. But I don't think it'd be overkill for the entire setting.