Bootflap,
I see where you're going with the momentum thing, but...
acceleration = force/mass
So for a given seat rocket impulse, the lighter bod on the seat = less mass = more acceleration.
The light occupant/seat combination will be going faster after the rocket impulse than a heavier one.
Momentum-wise, though, a heavier occupant/seat going at the same speed as a lighter one will have more momentum.
IIRC on the EA6 Prowler the seats were command-eject sequenced so that if one guy was lighter than the one next to them they wouldn't accelerate past their mate and torch them with rocket exhaust.
ST