This reminds me a lot of the calculations you got to do when playing the board game risk. Concentrate your forces too much, and your opponent can grab too much land which later will allow him to obtain more reenforcements. However if you spread your forces too thin, then you can get picked off too easily.
I guess in real life however significantly smaller forces isn’t as big of a problmem as you can fall back. You can trade land for time to regroup into larger armies.
I seem to remember from playing the computer game Total War, that one thing I learned there was the significance of being able to attack a retreating enemy. Since if it was too easy to run away, your victories meant little as they could just come back in force. Hence you really needed fast moving units like cavalry to catch up with a retreating enemy and make sure you could inflict some damage on them before they got away.