Before Basel Committee’s risk weighted capital requirements, if a residential mortgages and loans small businesses produced the same risk adjusted expected net margin/return on asset (eROA), these would produce the same risk adjusted expected returns on equity (eROE)
That was then: Now, with Basel III a residential mortgage with e.g., a loan to value of 80% to 90%, has a risk weight of 40%; which times the basic 8% requirement, results in a capital requirement of 3.2%, which allows banks to leverage 31.25 times their capital (equity).
While a loan to small businesses have a risk weight of 100%; which results in a capital requirement of 8%, which allows banks to leverage 12.5 times their capital.
So, if the eROA for residential mortgages and loans to small businesses is 1%, then residential mortgages produce a eROE of 31.25%, while loans to small businesses only a eROE of 12.5%
And so, even if interest rates on residential mortgages were reduced by e.g. 0.5%, resulting in a lower risk adjusted expected ROA of 0.5%, residential mortgages, yielding banks a 15.625% risk adjusted eROE, would still be more interesting to banks than loans to small businesses
And therefore, small businesses, in order to access credit, must pay higher interest rates so as to increase the expected risk adjusted ROA, in order to produce banks a competitive risk adjusted eROE.