I am not 100% positive but I believe Magnaglides use the same size core for both L and M so the only difference is the amount of thread wound on the bobbin core as there are no cardboard sides on magnaglides. I wouldn't be concerned about backlash because the magnetic core keeps it from happening. I am thinking that is the reason the supplier thought it would be ok to send the L instead of the M. Here is a link to their website. They are only open on weekdays but I am sure they or the supplier you got the magnaglides from can confirm this.
https://www.habanddash.com/