Why do alkenes with more surface contact have greater London forces? Why is it that alkenes with greater surface contact have greater London forces? I thought greater London forces were dependent on the size of the molecules, or the number of electrons, rather than the surface contact.
You are correct that London forces are dependent on the size of a molecule, but think about why this is. One reason is that a larger molecule tends to have a greater surface area than a smaller molecule, and thus there is more surface area available to interact via London (and other) forces.  

Also as stated in this Wikipedia article:  


  London forces become stronger as the atom in question becomes larger, and to a smaller degree for large molecules. This is due to the increased polarizability of molecules with larger, more dispersed electron clouds.