Mika's not all that much heavier, having a fairly slim frame himself, but he's also a vampire. Which means having several times the strength an ordinary human would.

In any case, he ignores Armin's indignant protest and starts off again. Though now that he doesn't have to let a human try to keep pace he can move much more quickly, easily leaping from one (hopefully) solid piece of ground to the next with strides faster than most humans could run.