X11 is a symlink to /usr/bin. So /usr/bin/X11 points to the same location as /usr/bin. The symlink takes up almost no space.
lrwxrwxrwx 1 root root 1 2008-01-15 05:14 X11 -> .
Way back when, people used to put X applications in /usr/bin/X11, and command-line tools in /usr/bin. It appears Ubuntu decided /usr/bin is good enough. But for compatibility with any scripts that might refer to /usr/bin/X11, they use the symlink to trick the computer into finding X apps even if they are referred to using the /usr/bin/X11 path.