Hello,
I said that airport size and the fact of being able to give vectors have to do (IMHO) with the ATC proposition to finish visually. Why ? Because generally in small IFR airports the IAF is in final which implies a procedure hippodrome (longest integration) and there's not a lot of traffic. So in this case and if the weather is good, the ATC is more likely to propose a visual app before the IAF (which is on the RWY axis) than at EGKK or EHAM, because there's a real interest.