Yes, people generally can get massive, crazy kill/death ratios on bridge battles, especially if they are "designed" in the person's favor. The best example of this is when I played as Hungary and loaded up an army of about 4,000, consisting of mostly halbadiers w/ some arbs and a few CS in Kiev to face the Mongols. Also my mighy eight command prince and a lone Avar Noble who had achieved the "famously brave" trait for +3 morale.

I wasted their army of roughly 17,000 troops when they tried to cross the bridge, although it kept me going well into the night to do so. I know that the AI wouldn't have achieved nearly such good results by itself. The kill/death ratio was something like 14 to 1.