If this is happening in all applications within XFCE4, I would say it looks like a DPI issue, however for me altering the DPI changed both the menus and the application windows, so I am surprised that only one of those is off in you case. You can have a look in Settings > Appearance > Fonts. My DPI is set to 160 on a 2160×1080 display (the UI is quite small with these settings but I like it).

There is also a Window scaling setting in Settings > Appearance > Settings, mine is set to 1× but I know some prefer having it set to 2×. Be careful not to set it to 2× if your DPI is already set to make the UI look larger, otherwise the windows won't fit in the screen anymore and it will be difficult to change the scaling back to 1×. If that happens, you can move windows using Alt+Click if you have a keyboard, or using the window menu (click on the left icon in the title bar).

If stuck after changing the above setting, you can also change the window scaling factor in Settings > Settings Editor > xsettings, and the option will be on the top plus you can navigate through the individual options using the up/down arrows of a keyboard if the options you want is out of the screen. The DPI settings are also in this settings page, if you scroll down.

If you are unsure, just make a backup of you ext4 file in the sailfish_ubu_chroot folder before altering the xsettings.