Regarding the less than ideal trim of loading bags on the 757 in holds 2 and 3 may I suggest it is to get a quicker turn-round as you have two crews loading two hold doors rather than one crew trying to get 200+ bags through one door into holds 3 and 4.
And yes, when I go on holiday my case is closer to 20 kg than 13. Multiply that by the capacity of the aeroplane and I guess your average charter jet is overweight.