New Delhi, Sept. 9 -- It's getting harder for small listed companies to break into the big league.

Fewer companies that debuted on India's small and medium enterprise (SME) exchanges have graduated to the mainboard than during the post-pandemic peak. The slowdown is not entirely regulatory. A flood of recent initial public offerings-with many companies still too young to qualify for migration-has pulled down the overall conversion rate.

The picture changes sharply when companies are grouped by IPO vintage. Of the 551 SMEs listed through 2020, 339, or 61.5%, subsequently reached the mainboard.

The 2018 batch produced the most graduates: 66% of 141 companies migrated. That was followed by the 2017 cohorts, in which 64.7% of 133 companies...