The accorion icon (accordion nestable) size can only be adjusted at ID level, when styling under a class there is no change.

Hi Tristan,
Welcome to the forum, and thanks so much for your report!
If you add a class to the icon, make sure to remove the default 1em from the ID. Since the ID always has a higher specificity than a class, the ID value overrides the value of your class.
Best regards,
timmse