that would benefit bigger armies and I for one want to get rid of that. It's much more exciting to have many bands of small units. In RTS terms, your suggestion would end up in one big battle but mine would have many small skirmishes.
It's a trade off. Large armies move on their stomache. A small raiding party can subsist off the environment in plentiful areas. A ten thousand man army needs an army of caravans supporting it. The same type of drawback goes for large formations. A phalanx is an impenetrable wall of shields and spears from the front, but changing facing is a significant undertaking and marching while holding that formation takes great care. Something like a hedgehog formation, while being ridiculous to assault from any direction, is also purely defensive as you can't really walk backwards or sideways in formation while armed and armored.
If supply and mobility are done realistically, big and badass isn't always the best way to go. You can cut the supply lines of that huge army with small raiding parties. You can flank and route the huge formation using significantly smaller forces so it can't meet them all head on.
