The components inside the frame are being laid out by layout managers. When you resize the frame, a layout manager has to do its best to lay out the components in the content pane. If the available space is less than the minimum size of your single component, the layout manager isn't able to tell the frame that it shouldn't have resized, so it does its best and makes the component smaller than the minimum you've specified.

If you had more than one component, one of which had a minimum size, the layout manager would respect that minimum size when the frame got smaller by reducing the size of the other components, as far as that was possible.