The length of the antenna does play into directivity but is not the only factor. For a dipole the height above ground, in terms of wavelengths, is actually more of a governing factor. Note that at 80 meters a dipole antenna would have to be extremely high off the ground in order for the ground effects to be reduced. At higher frequencies it is less of a problem.
In a vertical configuration a VHF and even an HF antenna can see improved performance by being longer than one might otherwise make it for resonance. What this does is adjust the radiation angle to put more energy in a direction that is useful.
At 80 meters a dipole that is 20 ft off the ground has a radiation pattern that would favor receiving stations that are overhead - unlikely to ever be the case.
A vertical VHF antenna for 2 meters might be 5/8 wavelength long and have appropriate matching sections to bring the angle of radiation closer to the horizontal - useful for ground stations. For overhead work (satellites, aircraft) a 1/4 wave might be more suitable.
ARRL Handbook and website has some good info on the relationship of height above ground and directivity.