To resize proportionally, one has to hold down the Shift key while moving a corner handle.
Unfortunately, this disables the snapping to smart guides: when view>snap guides is active, resizing a layer is supposed to auto-align to the edges of other layers, but it is not the case when doing it from a corner handle while holding down shift, a common practice.
Shift-resizing a middle handle does respect the wanted snapping behavior.
CTRL is the shortcut supposed to disable snapping, not Shift.