Circle size does not respond to constraints

[krzd]

I am adding the the control to the view and then set NSAutoLayout constraints, but no matter how large the view turns out to be the circle inside remains the same size.
I want this size to be different according to device/orientation, hence the use of constraints. But even the alternative of setting minimum circle radii does not seem to do anything.
Is/was this behaviour intended, is it a bug or am I using it incorrectly?