Unresponsiveness is still a factor of size. First, there are numbers: typically only of X% of clients will have problems. As a company, you can either fix that problem or ignore it. If you ignore it, eventually the customer will churn. Now, if the market is made of 5-6 mega-corps, as a churned customer you have only a few options for a migration. That will rise (eventually) your bar to churn out.

If the market was made of 2000 small companies, you have much more options and companies are forced to better interact with you. Also, with a smaller user base a user churning is a higher percentage of revenue loss so, they are even more interested.