SDDM login screen resolution

I have installed openSUSE Leap 42.1 on two PCs. Both PCs have a nVidia graphic card (proprietary driver) and the configuration files (xorg.conf, sddm.conf, etc.) are identical.
On a desktop, with an HD monitor (1920x1080), the SDDM login screen starts at 1920x1080.
On a laptop with a 1600x900 monitor, the SDDM login screen starts at a very low resolution (largely lower than 1600x900); after login, Plasma 5 starts at 1600x900.

Is there any way to set the resolution of the SDDM login screen?