To have 'little faith' is not a reference to size because faith is a state of being; faith is something you do. For example, you can't say 'that eagle is little flying and another eagle is big flying'. They are either flying or they're not.
In the same way it doesn't make sense to think of someone having 'little faith' and another person having big faith...
The puzzle is solved by understanding that the word 'little' is a reference to duration of being or short or brief or momentary... Peters' struggle with 'little faith' in Matthew 14 is recorded so we can receive insight into our own struggle to live by faith.
God did this so that, by two unchangeable things in which it is impossible for God to lie, we who have fled to take hold of the hope set before us may be greatly encouraged.
- Extract taken from "Meditation on Mysteries Vol. 1" by David Storer.