The reason battle field styles of jujutsu look different on the ground is less to prevent the enemy from biting you (or something equally distracting) than from having someone stick a spear in your back. Mobility and alertness, which require a fairly upright stance, were keys.
Some of the differences...
The older styles assumed and used a weapon. Ideally, the enemy went down, you slammed your full body weight on the point of your knee either to the back of his neck or his lower spine and then shoved a knife or short sword, his or yours, into his neck. In an un-ideal situation, where you were brought down, you tried to take him with you- which can turn into a scramble that looks a bit like later sport grappling, but with an emphasis on getting weapons into play or disarticulating the spine.
There are no submission holds- what we think of as submission holds are just stages in a disarticulation, quickly passed through.
Take downs are targetted, meaning that the force is either focused on to a bad place, such as the point of the shoulder or a bent neck, or is combined with a lock (all of your weight plus all of the bad guys weight landing on his locked wrist and shoulder is crippling). Or targeted in the sense that he lands on something bad... such as your raised knee across the lower back or nape of the neck. (Seeing a pattern here? When fighting armed men in armor, breaking the spine is the way to go.)
For want of a better example, look at the battle scenes in Braveheart and imagine trying to wrestle in a surging mass of armed men. Scared, angry armed men. And charging horse. Don't forget the 3/4 ton beasties with the sharp hooves. You can't stay down in that environment.
Which brings up another difference, time. A duel or sparring is as much a battle of wits and will as it is a test of strength or skill. But in mass battle, every second might bring on more opponents or give an archer a second to aim at you. You have to finish the enemy in front of you, now, because the rest of his clan is right behind him. And if you don't finish him befor moving on to the next, he'll be crawling after you, to stick a knife in your knee while you fight with his brother.
Okay, that was long winded.