Apparently (and I am no expert!) the rocket that provides ejection assumes a certain centre of gravity (or can adjust accordingly for the weight of the 'passenger' in the seat) ..... if you fall outside the weight parameters there is a possibility of the centre of gravity not being accomodated properly and the whole works can spin as it ejects .... (some one said "like a catharine wheel") which impedes the parachute lines....probably makes one a little dizzy in addition to the significant compression that all those 'g's will apply (particularly to a 'large' guy - or gal!).
(speaking of Spanish gold -- betcha all that high speed spinning will empty your pockets of any change you might have -