"It has been, now of a long time, observed by others, that all sorts of
heavy bodies (allowance being made for the inequality of retardation which
they suffer from a small power of resistance in the air) descend to the
earth from equal heights in equal times; and that equality of times we may
distinguish to a great accuracy, by the help of pendulums. I tried the
thing in gold, silver, lead glass, sand, common salt, wood, water, and
wheat. . . . I observed them to play together forward and backward, for a
long time, with equal vibrations. And therefore the quantity of matter in
the gold was to the quantity of matter in the wood as the action of the
motive force upon all the gold to the action of the same upon all the
wood; that is, as the weight of the one to the weight of the other; and
the like happened in the other bodies. By these experiments, in bodies of
the same weight, I could manifestly have discovered a difference of matter
less than the thousandth part of the whole, had any such been . . ."
Thus Isaac did worry about and even asked nature if, and then just how,
the gravitational force depended on the quantity of matter (inertial
mass). He concluded that the two properties were in the same proportion
in all observable objects - but it seems he saw nothing remarkable in this
(I think he "expected" it).