I made a simple form with three QPushButtons on a horizontal layout with style sheets like this:
QPushButton {
border: none;
background-color: red;
min-width: 100px;
min-height: 50px;
}
(colors and min-heights are different). The full project is attached. Layout spacing is set to 0.
On Linux and Windows, it works as intended (see linux.png). But on Mac OS the buttons overlap (see mac.png).