Cashflow per door for larger properties vs cashflow for less doors

[James Wilcox]

Looking at cashflow only, is it very common for the cashflow per door/unit exponentially go down as you invest in properties with more doors? For example a fourplex that might yield a $100 per door (4 doors) is higher in cashflow per door then a 28 unit apartment building that might bring $20 per door. (made up numbers)

Why does the 28 unit not bring close to the $100 per door? Is there a number that is too low per door for larger complexes (minimum cash flow requirement)?