Originally Posted by The Glyphstone
Lexicanum says a battlefleet is usually between 50 and 75 ships, up or down depending on the relative threat level. So it's only, at worst, a 6-1 margin without local support ships. Though I understand the Battle of Reach did have at least one Supercarrier there, so the largest Covenant ship in play will outclass the largest Imperial ship in play if they each get a full fleet to play with. The average Imperial ship size is far larger than the average Covenant ship size, though, even with a supercarrier dragging the median up - the Lunar cruiser's apparent opposite number is the 1.7km CCS Battlecruiser, and the smallest ship the Imperial Navy deploys are escort-class destroyers at 1.5km.
Thought for sure I had read a Battlefleet consisted of only a dozen ships or so. Maybe that was for individual systems, not subsectors.
Kinslayer brings up some good points too, and I think it'd be worth probing the issue of fighter craft a little more, or rather more pertinently, boarding craft.
As mentioned, the optimum Covenant engagement range is less than a quarter of that of IoM cruisers and capital ships, which means that any covenant craft which actually make it into firing range, are also well within the range of any smaller Imperial Craft to board either by teleportation or the various range of assault craft the Imperium has access to.
Once you've got Imperial boarding parties on Covenant craft (whether that's Cadian Shock Troopers or Space Marine Terminators) they are sure to carve a bloody swathe through the Covenant crew, as we've previously established that standard Covenant forces (grunts and jackals) just aren't going to be a match for battle hardened IG assault troops, let along the Emperor's chosen harbingers of destruction.