You have to understand that what changes is the perspective, not the scale.
when you ask
In this case, the right cube should be 750px closer to the user, so why is it not considerably larger than the one on the left?
What you need to think is that you are 750px closer to calculate the perspective, NOT THE SIZE.
The key is this part in the specification:
Each 3D element with z>0 becomes larger; each 3D-element with z<0 becomes smaller
Your cube is centered in the origin (You are giving each face the similar movement, in oposite directions). That means that the cube center (ok, there is nothing there, but get the concept) will keep the same size.
Similarly, if you imagine the z plane, it cuts the cube more or less in the half. In that plane, there is no dimension change.
The front face does indeed grow larger, and the backface does indeed grow smaller (by small amounts). This is more easily seen in the lines that get in nearer vertex, or in the farest vertex