Cannot wrap my head around it but each and every android smartphone I own outputs volume much louder from the Airpods.
There must be some limitation within iPhones, every android I own cannot possibly have sound output levels that much higher.
Thinking Apple needs to look into this issue. Perhaps it is a software problem. Obviously airpods have a big enough drivers as evidenced by output volume from androids.
By any chance does your iOS device have Volume limit and/or sound check toggled on in the Music settings control panel?