If you create a widegt with many QSpinBox or QDoubleSpinBox in a horizontal layout, you'll notice that QDoubleSpinBox's minimal width are way bigger than QSpinBox's. So that it makes your GUI very large if you add many of them.

In the code, QSpinBoxPrivate calls setLayoutItemMargins(QStyle::SE_SpinBoxLayoutItem) while QDoubleSpinBoxPrivate does not. That may be the cause of the problem.