more about Mars magnification

In article , Mick wrote:
At present Mars should be over 1/2 degree at 100 X's, which is a full moon
disc. At 200 X's, Mars should appear
as the moon would at 100 X's. This just doesnt happen because the moon
takes up the whole field in the EP at 100X's

So there is some optical trick occuring that eludes me...

For Mars to have the same apparent size in the eyepiece at 200x that the
Moon has at 100x it would have to be 900 arc seconds wide instead of
20 arc seconds wide.