The dipole moment µ of a diatomic molecule is a measurement of the separation of two opposite electrical charges.
The magnitude of the moment is equal to the charge q multiplied by the distance r between them.
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"CH"_3"Cl" and "CH"_3"F" are not diatomic molecules, but most of their dipole moment is caused by the polarities of the "C-Cl" and "C-F bonds".
Here's a simple argument to account for the differences in dipole moment.
"F" is more electronegative than "Cl", but the "C-F" bond is only 72 % as long as a "C-Cl" bond (139 pm vs 178 pm).
Thus, the charge separation in a "C-F" bond would have to be 39 % greater than that in a "C-Cl" bond to generate the same dipole moment (0.72 × 1.39 = 1.00).
The "C-F" bond polarity is not great enough to compensate for the shorter bond distance, so "CH"_3"F" has a smaller dipole moment than "CH"_3"Cl".
