I have a delta printer with a round print bed.
Rather than use the radius of the print bed to define it's size, Simplify3D uses size of the square contained within it.
What this means is that when you try to 'Scale to Maximum' an object on the print bed, it is only maximised to the size of the square and not the actual size that would fit on the round print bed. For example, a long skinny model is not maximized to take up the full width of the bed and a round model is not maximised to the full radius of the bed.