First aid kits only need to be basic – you’ll not be performing heart surgery on someone!
Have a think about what major injuries you might come across on any ride:
Blood loss – Bandages bandages
Heart attack – Aspirin for the person to Chew on while you wait for Paramedics
Broken bones – Sam splint but only useful for certain breaks and not always the easiest to pack.
Head injuries – well not much you can do other than stem any blood loss and comfort them until help arrives.
That said, in my first aid kit is numerous bandages a vial of saline, some compeed and some zinc oxide tape.
Having knowledge of basic first aid is infinitely more useful than anything you put into a first aid kit!