Perhaps you pronate slightly? Try adjusting your pedal stance (washer, or move cleats inward bit by bit to test) to see if a wider stance reduces the outer foot pressure.
Insoles are often overlooked as well. Good arch support will help redistribute the load under prolonged power and should help provide relief.
As someone with massive arches, I’ve always had a tendency to pronate- wider pedal stance + proper arch support remedied this for me…