F= G (M * m) / r^2
Where M and m are the masses of the bodies affected.
Also, Newton's laws of motion (he was a clever guy) show that force is also proportional to mass:
F = m a
If we equate the two forces we see that both sides have the mass, m, which cancel each other out.
So, the acceleration that the body experiences is not proportional to its mass.
This is not true for a Universal force. The strength of a universal force is not linked to the mass of the body and so when we equate it with the law of motion we get something like this:
m a = U
Where U is the arbitrary universal force.
So the acceleration experienced by the body is now (inversely) proportional to its mass.
Maya copies nature in this regard, so if you want to use variable masses to drive your simulations, be aware that the Gravity force will affect all the particles uniformly but a Uniform field will affect them in (inverse) proportion to their mass.