Glacier speed depends on two things: Sliding at the base (friction) and internal deformation (breaking and reforming bonds, imagine how honey flows).
Q. What could decrease the amount of friction at the base of a glacier?
A. 1. Adding Water
2. Adding Mud
3. Increasing Slope
Q. What could make ice deform faster?
A. 1. Warm the temperature of the ice (I know this sounds odd, but warmer ice moves faster, like how Silly Putty is easier to mold once you warm it up)
2. Increase the slope
3. Increase the weight on the ice
So, bigger glaciers can move faster than smaller glaciers if they are on steep slopes, have mud or water at their base, are made of warmer ice, or are really thick! But size alone does not determine speed.