alt Hacker NewsPlanes typically have their center of gravity just forward of the rear wheels. This makes it easier to rotate on takeoff.
The margins are thin enough that certain planes will sometimes have people in the back get off first, before the people on the front, to avoid tipping onto the tail like this.
I have been on small aircraft where some of the larger passengers (me, at about 100kg, and a couple of other guys my size) were asked to sit a little further back for the takeoff but move up front and get off last because of the difference in balance as the fuel was used.