the average Macdonalds in the US enjoys about $2.3m in annual sales. The average BK is about $1.3M. So when restaurant A sells $1m more than restaurant B you are going to see differences in service, marketing, renovated buildings, etc. And cleanliness is often a reflection of having more labor available to keep the place in shape. I believe that even low volume restaurants should be clean but the challenge is much greater due to labor costs.