You mean the background color "overspill" to right & bottom? I don't know but you might play with border-... or padding-... (I'm thinking margin- does not use background color, but I might be wrong) having a width/transparency/grey....
QCheckBox inherits from QAbstractButton, which provides a hitButton method that controls which parts of the button are clickable. All you need to do is make a new subclass of QCheckBox and override hitButton so it always returns true:
class BigHitCheckBox : public QCheckBox
bool hitButton(const QPoint & pos) const override
This is much simpler than the solutions previously mentioned, and it should play well with the hovering, disabling, and tristate features of the checkbox without any extra effort.
By the way, this makes the entire checkbox widget clickable, but it doesn't make other widgets outside the checkbox clickable. I don't think the OP needed that.
Nevermind - in the end it turned out the label was modeled as a separate QLabel instead of the text of the QCheckBox. Once I removed the label, and put the text inside the checkbox, I got exactly what I was asking for.