I think the poster may be confused because how big the molecule is can play a role with the dispersion forces and is mixing it up with the mass. For example, if you have a straight chain pentane vs a straight chain heptane, which one would have a higher bp/mp? Heptane, because its bigger and has more dispersion forces.
@DrRoyal Pains: MP/BP depend on the intermolecular forces. As the above poster said, even though ionic compounds may have less mass, they still have really high MP. So look for intermolecular force and use that for determining if higher or lower BP/MP. Then, look at the molecules you are comparing. If you have say straight chain butane, pentane, hexane, heptane, then heptane will win, not on the basis, "oh because it has more mass" (having more mass is true but there is more to it than that). The reason would be because of the fact that its bigger and hence has more surface area for dispersion forces.
Some other problems may require you to pay attention to the branching as well.
Hope this helps!
