Physics question. Please answer using physics and math?

Pythagorean theorem. If they go at the same speed you have that the horizontal displacement of the dog is proportional to the length of the animal. Having a larger length will allow for a larger distance by step vs. having shorter legs. the horizontal distance traveled by each paw each step will be L*sin(O) ; where L is the length of the leg and O is the angle the leg moves forward from a vertical axis. So assuming similar range of motion (similar angle) for both dogs, the only variable left is L, for larger L less steps for X distance and the opposite for shorter legs.

What is your evidence that this is true? Cite your source. You've made an assertion but without evidence to support it.

Maybe they stride with the same frequency of steps. I wouldn't be surprised.

You must see the step size. Is it the same? Now calculate the distance per second. What is its value.