Larger creatures require more effort to move.
Thats assuming they are less efficient at movement. If efficiency remains the same, size won't matter. They would exert more energy in each stride, but cover an equally proportionate amount of extra distance.
Yes, they require more effort to move, but that effort moves them a greater distance, in theory.
Of course, since the term "giant" is such a loose one, its hard to make any claims about the efficiency of said movement. But such a claim is just as hard to make against a generic medium size as well.
Unfortunately, the current design doesn't afford that granular level of detail. In an ideal model, each race would use movement based on their own efficiency. The alternative would be to keep all races at a constant efficiency and vary the amount of energy given.
Let me wrap it up by saying that this is far more thought to movement by size/race than is given in the code.
Point I am getting at is that applying logic to an apparent mechanism in a MUD isn't often sound. Assuming any level of realism usually ends poorly. In many ways realism will be mimicked, and you will find the ghosts and shadowed influences of logical outcomes, but its certainly not a physics engine driving the game, for instance. The movement difference could be an attempt to mimic actual physics, or simply a game mechanic to promote balance, or further just a point of design that is not complex enough to fully, and accurately model the behavior it represents.
Just something to chew on when you are ever in the game and think about why X doesn't work like it would in real life, or how you might think it should work.