I personally think that he chose that title due to the fact that Lennie is seen as smaller due to the fact that he had some sort of special needs. Furthermore, it could be titled in this way because everyone else in the story seems to see themselves as greater than him due to the fact that they are "smarter". I think the role of the mice in the story represents how others see Lennie. Where Lennie sees the mice as something special and sweet, others see the mice as gross. This puts emphasis on the fact that Lennie does not see what is wrong with himself, where it is blatantly obvious to those around him.
Side note: I hope you loved the book as much as I did, it is incredible.
