Mass and balance can be a problem for small Cessnas. Mine is 726 kg max T/off and Ldg with empty mass of 518.5 kg plus 2 pilots at say 80 kg each = 678.5 (full fuel at 98 lts max with 0.7 sg) gives you a limit of 47.5 kg or about 2 hours plus some reserve. decreased by adding to mass with headsets, bags spare oil, hi vis tabards! etc and you are over the limit. However as the pendulum effect of cabin below the wing is fairly stable most people do not realise the situation and fill up regardless. Try to fly with skinny light instructors (not me I am afraid). I flew with an examiner the other day at a little over 65 kg!! IDEAL for fuel anyway. To stay absolutely legal refuel after each flight and stay local!! Unless you are solo?