A couple of problems, there...

You're assuming that all the sides are of equal length. They aren't. You're either cherry picking a side or are making some supported assumptions.

In your drawing you are showing thicker stones at the top. This isn't possible from a materials standpoint... the stone would weigh so heavily on the bottom stones that the casing would shatter easily. This would Not Be A Good Thing and would not bode well for the workers.

Finally, you keep ignoring that the angle of the pyramid's slope actually changes.