In the draft library staffing models [1], the Access Service Manager I position has fewer FTE (full-time equivalent positions, where 1 FTE = 40 hours per work week) direct reports than the Access Services Manager II does. The distinction has to do with amount of direct reports rather than size of library. Also, please note that the ASM I position is more entry level than the ASM II due to the number of direct reports.
Links:
[1] http://my.metrolibrary.info/drupal/sites/default/files/documents/sample-library%20staffing%20models.pdf