B737 manual states that wing tanks must be full if more than 800kgs in the centre tank, although, as previously mentioned, this can be varied as a result of the Fuel pump AD that is ongoing.
My money's on wing bending stress upward for the ZFW (sorry mass) and wing bending stress downward plus localised upward force from the gear attachments.
Interestingly, when you go from a 300 to a 700 series the wing changes and so does the fuel capacity in it(the 700 carries 1.4 tonnes less).
The 300 zfm is 48307kgs, and its mlm is 51709, a difference of 3402 kgs.
the 700 zfm is now 54657 and mlm is 58604, a difference of 3947 kgs.
The increase in zfm is 6350 kgs, the increase in mlm is 6895.
Perhaps the greater increase in MLM is due to the lesser amount of fuel in the wings for a max weight landing i.e. less bending at the root compared with the 300?
Ps...Alex...are you still doing courses at the Gatwick Hilton, and have you still got the dodgy XJS?