If they have different weights, the higher weight substance tends to have a higher boiling point because of London Dispersion Forces depending on the polarizability of the electron cloud (More weight, more electrons).
When you compare the secondary and tertiary amine of the same mass, what is important is the polarity of the molecule. The electronegative difference is greater between the N and the H vs. the N and the C. That leads to a slight increase in polarity. However, amines are special because H-N bonds mean that the dipole is particularly short and you can have Hydrogen bonding between the molecules (a particularly strong dipole-dipole interaction - like in NH3 and H2O and HF). As a result, the secondary amine is much more attracted to itself in the liquid phase than the tertiary amine.
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