When it comes to business banking, the bigger the business the happier the customer. Medium and large businesses reported higher satisfaction levels than micro and small businesses in two recent surveys.In the latest DBM Consultants' Business Financial Services Monitor, released yesterday, National Australia Bank gets a rating of 7.6 from its large business customers (turnover of $50 million or more), 7.4 from its medium business customers (turnover of $5 million to $50 million) and 6.8 from its small business customers (turnover of $1 million to $5 million) and micro business customers (turnover of less than $1 million). The DBM scale ranges as high as 10.ANZ and Commonwealth also scored better with large and medium business customers than  with small and micro business customers. The exception was Westpac, which recorded the highest scores with micro and small business customers, but lagged its peers in its large business customer score.Roy Morgan Research's Business Bank Satisfaction Update, published earlier this month, reports that micro business customers of the big banks are the least satisfied of all business customers.While 58.8 per cent of NAB's micro-business customers reported they were satisfied with the bank, 66.5 per cent of large business customers said they were satisfied. According to Roy Morgan, businesses that gave low satisfaction scores to their banks did so because they felt the bank had a poor understanding of their industry, did not have a good knowledge of the business and failed to keep in touch.Taking the overall results of the two surveys, Westpac emerges as the highest-rating business bank. In the DBM survey it had the highest satisfaction scores among micro, small and medium customers. In the Roy Morgan survey it had the highest scores among small, medium and large customers.