Many military forces only authorize so many officers of any given rank though. So while many, many LTs might reach a point in service where they can be promoted, it might not be possible due to an absence of slots. I would expect lower LT, higher LT, and Captain to be something of a given for gradual promotion, but jumping to Major is not a given, there have to be slots open to ascend to that rank, and then but the time you get to Major, newer schooling seems to come into play too. Majors and Lt Colonels, Colonels and Brigadiers, and the other General ranks in US service all require a different school be attended before they are even considered promotable to the next rank. You can get to be a given rank, but you can't get beyond it without taking the classes for that rank first, and after Lt. Colonel, it appears you have to have the classes nearly immediately, because there seems to be a rush to move a newly promoted officer out of his current slot and into the classes.
In times of war, there are more slots open, and the Inner Sphere at least was pretty constantly at war for 300+ years. Not always, no, bu they were with at war or preparing for another, so "bloated" military structures were likely the more normal situation. And when "real" peace set in, many of those lower ranked officers were not encouraged to remain in service, and would be released, and allowed to join a local guard or militia unit instead.
Long story short, the number of offices needed to fill any given quota or rank level would be factored into recruitment over a period of time, and more or less would be accepted to avoid having too few or too many around at one time.