I think you're right. When push comes to shove size has consequences. Specifically, the bigger the animal, the bigger the torque on every movement. This means bigger animals move slower (more accurately: "can't change speed"). In the case of Woolly Mammoths and dinosaurs ... a
lot slower. Their turning radius, certainly at speed, will be something like that of a truck.
This means that whilst outrunning a T-Rex might be difficult, it should work like a bull: you have zero chance when running away from a bull that's trying to attack you. So don't. However, if you just avoid the bull by going sideways (walking speed is more than enough, there is no real need or the showy stuff toreadors perform at bull fights) and let it attack you until it tires (and collapses), at which point you calmly walk over and kill it with a pointy stick ... yes it takes some practice but it's definitely going to be how such fights would go in practice.
Worse: it's not like humans are the only animals that are capable of figuring this out.