Education Review Office figures indicate small schools are more likely to run into problems.
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The number of schools needing frequent checks because of concerns about performance has been falling steadily.
The office visits 190 such schools every year or two and says, on average, they have about half as many pupils as the national average of 220 students for primary schools and 640 for secondary schools.
In contrast, the 450 most trusted schools, which are visited only every four or five years, are bigger than average.
Principals say the difference is probably because large schools have more staff to share responsibility and ensure key jobs are done well.
The number of schools doing so well they need less frequent vistis has been increasing.
The office has not said why that has occurred, but education experts say it is because schools better understand what the review office wants, school trustees have more training, and there is more support to help schools improve.