Every application has a minimum window size. For Xmind this is something like 26 x 18 cm on my 1920 x 1080 screen probably limited by the icons just as Christophe said. There are two full screen modes (View → Presentation and View → Brainstorming) but no minimal views (e.g. toolbar-less view). But 2/3 of the screen? Are you sure you have squeezed it to the minimum? OTOH, you could have multiple displays, one showing Xmind, another your Powerpoint/video/whatever.