NCR High Command and its implications
These are the two excerpts we get about NCR High Command
"You will not be assigned replacements until High Command can figure out where the hell to pull replacements from. You have my sympathies, Major. Signed, Scheppman, Andy Assignments Office." - Forlorn Hope Mandate
"I've seen the reports of this so-called "Caesar's Legion". It sounds like they're just another bunch of raiders, but the higher-ups back west seem to think they're a serious threat." - NCRF Terminals
With these combined we can see the NCR High Command is both afraid of the Legion and is struggling to find men to send to Forlorn Hope. I also think the implication that the higher ups are struggling to find men to send to Forlorn Hope is also an implication they cannot find men for the Mojave. I also think it is important to consider that if the NCR army was quite large in the home territories the NCR high command would have no reason for fear.
That is not to say they couldn't raise more men, but I believe the current army size is constrained by the senate. Just like the real U.S Government.
"He mustered the troops as many as his senate would allow and sent them in. All the right reasons, done all the wrong ways." - Caesar
But where is the rest of the NCR army? I believe they are controlling the other 300 miles down the Colorado river across from Legion territory, Baja, other territories, and the rest of NCR territory.
Let me know what you think of this interpretation if you believe it is the intended one by the developers or if there is evidence of large NCR reserves elsewhere.