Maybe it's unfinished feature or maybe related to tablet mode. I really don't know why is that. I checked too and see when you change the taskbar icons to small, the height of the taskbar shrink in only one millimeter. Don't forget that this build and another are beta.
I have the main taskbar double height so time-day-day show up in notification are. If I switch to small icons I notice what cuteee mentioned, very small reduction in taskbar size. This is with a clean install of 10056. @cuteee - tight on - pre-release software.