Helen Morrissey, pension specialist at Royal London, said the phenomenon could be a timing issue.
She said: "Auto-enrolment was carried out in stages with larger employers going through the process first.
"As a result they have had more time to communicate the benefits of pension saving to their staff. They will have also gone through the re-enrolment process – where you auto-enrol people who have previously opted out every three years – which will have increased numbers.
"Smaller employers have yet to go through re-enrolment but we should see numbers increase once this starts to happen so the gap should decrease.
"However, re-enrolment on its own will not be enough to bridge the gap and smaller employers will need more support in communicating the benefits of pensions to their staff."