: If our respective territories are similar in size, how come the Covenant's
: territory encompasses the homeworlds of the Prophets, Elites, Grunts,
: Jackals, Drones, Brutes, and Hunters, while we're on our lonesome? Did the
: Librarian decide that Humanity needed Lebensraum?

No idea. To be quite blunt I think it was just another way to drive up post war tensions between the Elites and the UNSC and hence better justify the resumption of hostilities that we see in Halo 4. (Along with "lying Humans") However in the process it does imply that Humanity's rate of growth and extent of pre-war colonisation was something that the Elites found to be erroneous in the known galaxy due to all the focus it gets, and hence is something comparable to or even exceeding their own territory and rate of growth. Hell, they seem more worried about it than they are of the Brutes and Prophets, and freely compare Humans to the Flood even though they should know better about that.

: Actually, I think that we're supposed to think that the UNSC's fleet is
: larger. You don't hear the Humans worrying about the deplorable state of
: the navy.
: The Covenant suffered some major losses in ships toward the end of the war
: from Cole's Last Maneuver, from Admiral Whitcomb's suicide run on the
: Unyeilding Heirophant and from the NOVA bomb. But High Charity's escort
: was the largest fleet that anybody had ever seen. I think the fact that
: the Covenant had that many capital ships in reserve speaks volumes as to
: the overall health of their navy.

In addition to the fleets on patrol within and on the edge of Covenant space, along with the defence forces for major worlds like Sanghelios, Decided Heart, etc. There was also a an old 2001 interview with Jason Jones where he said that the loss of the Fleet of Particular Justice, 314 ships, from the front lines in no way impacted the Covenant's search for Earth due to the sheer size of its navy.

Meanwhile all that Lord Hood had to stop what he thought at the time was Truth activating the Ark was 3 Frigates. I think stopping Truth would have warranted an all out attack at that point, yet he could only muster those 3 ships and a few Longswords. I think that speaks volumes about how Humanity was in Halo 3.

: As for dealing with the Infinity, I recall a short passage in First Strike
: detailing the absolutely comprehensive defenses around High Charity. Why
: wasn't the Sangheili homeworld similarly defended? Or defended at all?

There was actually a reference to Sanghelios being effectively impenetrable on page 453 of Glasslands. However the context was Naomi planning to infiltrate in order to pull Phillips out should he get into trouble, so it could just mean impenetrable in that sense - as in, Human spies and covert ops are not likely to sink into the background too well. Of course it could also literally mean impenetrable to any form of Human attack as well, and/or to any method we have of trying to get in undetected in the fist place. Either way though, it certainly didn't look that way when Port Stanley was blowing up their own ships in low orbit whilst remaining undetected the whole time. Whether resistant to stealth or force, it sure didn't seem impenetrable in the slightest.

The Elite's focus in the post-Halo 2 setting seems to have been altered in some way recently. In Halo 3, and even in The Return, the Elites were suing for peace with Humans to focus on the Prophets and the Brutes. Now however that seems to be undone and Humans are back in the spotlight (With the Brutes/Prophets being a secondary concern if even that). Obviously a Sangheili Empire that could blow post-Halo 3 Humanity away like sawdust becomes a problem if you want to have the Elites being antagonistic again, so I think that is possibly a contributing factor to the tables being turned by so much, so soon; the fact that the status quo has somewhat returned.

: Maybe Leviathan is right. 343i, and the writers they're hiring, have a real
: problem with "Because the author says so" creeping into the
: stories they create.

I don't think I've seen his post about that. What do you mean?