Created attachment 8136 Screenshot with the mouse cursor positioned over the always-wasted screen real estate The "Default Config" for the Xfce panel(s) wastes real estate. When running on small screens this becomes much more apparent, but it's still bad in principle. I suggest combining the top and bottom panels into one panel (which has 100% Length). The bottom panel in the "Default Config" only uses 10% Length, which is pretty pointless unless you want to see a tiny bit of the bottom of your desktop background. Even maximizing applications doesn't use up that screen space. It's always wasted unless you customize your panels.
I also suggest that, after combining the 2 panels into 1, that the 1 panel live at the bottom of the screen rather than the top. Most users (except MacOS users) expect the "application menu" to be on the bottom... but honestly it's not that big of a deal and I can live with it being at the top.
This has been fixed in the latest panel release with the updated config. The lower panel now intelligently hides, hence no more wasted screen estate. https://git.xfce.org/xfce/xfce4-panel/commit/?id=f586a3c99228596ce3c12d8e883c16e10e1b2ba4