How to make a subview fill its superview
Seems you misunderstand the frame and bounds, revise your code to To.frame=ParentView.bounds; should be OK.
Related: Why is there an frame rectangle and an bounds rectangle in an UIView?
Fix "20 points" difference from bottom in subView and View in Swift:
To.frame = NSRect(origin: CGPoint(x: ParentView.bounds.origin.x, y: ParentView.bounds.origin.y-20), size: ParentView.bounds.size)