Just to clairify, your argument is that you can't see the Covenant's entire armada, nor the UNSC home fleet in one condensed area? During a global, weeks long campaign? If the skirmish for Anchor nine was a decisive factor, this argument would make sense. However, it was not. The Covenant cleary didn't suspect anything of the forces stationed at anchor nine, else they wouldn't have been slipspace bombed so easily. As such, what you saw cannot possibly represent their entire scouting force.
Oh, and I must remind you, this is a battle for a planet, fought over the course of a few weeks. As such, both armies respective forces are spread around the surface of the planet, and the vast gulf of space surrouding it. Thus, even in the heat of a climactic battle, you can't possibly expect to see both factions' entire fleets all at once. In Halo Reach, you were treated to several small scale skirmishes, not the entire damn battle, which isn't nearly enough to make estimates based around. There could have been hundreds of vessels all in separate engagements. As such, you can't expect to see the entire fleet, only parts of it.
In Halo 3, the size of the defense grid around Earth is cleary established. However, you only see one percent of it. Guess why? They're spread across the planet, just like Reach's fleet.
You may have noted that I'm explaining this like I'm explaining something to a total moron. I'm using simple words, and repeating myself. Don't get me wrong, I have no intention of insulting your intelligence. This is just to ensure that there's no miscommunication whatsoever. I hope this doesn't get messy.
[Edited on 06.26.2011 1:15 PM PDT]