As the ranks are from the French, reversing the order of the adjective and noun can often give a clearer English meaning. A General officer is one who is in charge of a wide variety of units, rather than a single unit (cf Attorney General). That is more its history than its use now.
Private – private = individual/separate soldier, not in charge of anyone else.
Corporal – as in, “body.” The head of a less-formal body of soldiers, such as a fireteam. Corps, meaning the whole body of soldiers is of similar derivation.
Sergeant – related to “servant,” used in the sense of one who carries out the orders of higher officials. This would make sense in a military context, as it is the NCO’s who make sure that the orders of officers get carried out. Sergeant-mojor is of course a major sergeant.
Lieutenant – think “in lieu of.” One who stands in the place (tenancy) of another – a representative. In the military, it means “next in rank,” as lieutenant colonel.
Captain, as in cap = head. The head of a company.
Colonel - the head of a column of soldiers, a regiment.
Squad 13 (8-14)
Company approx 200
Regiment – four battalions (4,000)
Expeditionary Force 60K+
Corps – currently 200K USMC