And yeah, I mentioned it a little earlier, but where it comes to space combat, the Imperium has the Covenant beat in all categories but one: weight of numbers.
From what I've read, standard Covenant Naval groups consist of several dozen ships (capital, cruisers, and frigates), whereas the Battlefleet for any given Imperial subsector is generally going to number no more than a score of capital ships, cruisers and frigates. The Covenant fleet that took the Battle of Reach was 300+ ships, and while that was a substantial engagement, it's only reasonable to assume that this would be a conflict on a similar scale. That means even supplemented by non-ftl support and system defense craft, the Imperials are likely to be outnumbered 10 to 1.
All that said, Glyphstone's point still stands and then some. An Imperial Retribution class Battlecruiser is almost twice the size of the largest covenant ships, and moreover it boasts over 5 times the raw tonnage, and practically 10 times the firepower.
It's like comparing a P-51 Mustang to an A-10 Thunderbolt. The A-10 is only about twice as big, but it's in a completely different scale (and era) in terms of firepower and durability.