I think one thing that's being missed is that UNSC put most of its resources into the Navy; in fleet battles, it's stated that the most significant ground forces were stationed on Earth and Reach, but for many colonies a naval group was the only representation of military force around.

If you look at the logic of the war, where space engagements took precedent over ground engagements, it wouldn't make sense for colonies to have large armies, but it would for them to have a naval battlegroup.

UNSC ships could have crews in the thousands, but the backbone of their fleet were frigate type vessels, which only needed a few hundred crew. Even a small "army" of several divisions would need tens of thousands of personnel. So I think the better figure to look at isn't the total number of military personnel, but rather the number of ships the UNSC Navy could field.