If he couldn't get out of bed, how did they weigh him?
where:
F is the force between the masses,
G is the gravitational constant,
mf is the mass of the fatty,
m2 is the second mass, and
r is the distance between the centers of the masses.
m2 would be the earth's mass, r the distance from the centre of the earth to the centre of the fat guy, G is known (6.674×10−11 N m2 kg−2) and the gravitational pull between the two bodies was known so from this they were able to calculate mf, mass of the fatty.