Cap Rates for Large Apartment Complex are Lower. Why?

[Jerry Fung]

I am a new investor in New Haven, CT.  I notice the cap rate for large apartment buildings (50+ units) tend to price at much lower cap rates than smaller apartment buildings (4-10 units).  I'm referring to cap rates in the range of 5% to 5.5% for large apartment buildings vs 7.00% to 8% for small apartment buildings.  I am comparing buildings in the same neighborhood and in similar condition.  Any ideas why this is the case?