Its got nothing to with how an optic is powered. MOA has been part of rifle scopes since the first optics were micro-adjustable.
MOA is an angler unit of measure, For the fun of it lets go with a reported value of (6) MOA. Keep in mind that (6) MOA is its own value. So 6MOA is 6 MOA at all distances. Since the linear unit of measure changes (yards, feet, inches) the linear numbers adjust to the proper scale.
As an example MOA never changes, it stays at (6).
The reticle is designed to be a dot, and that dot measures 6 MOA.
As NKD pointed out, 1 MOA = 1.047" @ 100yds. So:
6 MOA at 100yds is 6x1.047= 6.282"
When the linear value in yards changes, the inches scale up/down depending on the yards. In this example, the inches scale down.
50yds is 1/2 of 100yds, 6MOA did not change. So 6.282"/2= 3.141"
50yds 6 MOA = 3.141"
25yds is 1/2 of 50yds. So 3.141/2= 1.5705
25yds 6 MOA = 1.5705"
For 10yds, just move the Decimal one slot to the left. 6.282 10yds = .6282"
10yds 6 MOA = .6282"