Why do densities of haloalkanes decrease when the size of alkyl group increases?
when the alkyl grp increases, the van der waals force between polar molecules becomes stronger, so shouldnt the molecules pack closer to each other and the density increase?

Here's what I get.

Explanation:

Your argument makes sense. For example, the density of C₅H₁₂ is about 0.63 g/mL, and it increases gradually to a limit of about 0.83 g/mL.  

With the lower alkyl halides, the halogen atom is the major contributor to the mass of the molecule. The density of chloromethane is 2.2 g/mL.

As you increase the number of C atoms, the Cl atom becomes a smaller part of the molecule. The alkyl halide becomes more like an alkane.

By the time you have reached four C atoms, the density has reached a limiting value of about 0.89 g/mL.