Peas in a Pod

"...like two peas in a pod." Where did this cliche come from?

It's supposed to reflect closeness between two people.

"Those two are like two peas in a pod."

Meaning:

"That couple of individuals coexist comfortably together as if they were a pair of adjacent seeds within the confines of the pod of a blooming Pisum sativum plant."

However, in most cases, pea pods (at least those most likely to be picked in harvest) consist of at least three peas...usually more. So, why is the phrase constricted to just a pair? A pair of peas in a pod reflects immaturity or incomplete growth.

Shouldn't it rather be a phrase for groups of about four people who are closely knit?

What is it about those two peas that stand out from the rest of the pod? One of them is certainly situated in the middle of two. Does that pea more closely associate with the one on its right than with the one on its left? What about the poor pea on the end?

I often wonder how sayings like this originate.

I also wonder if there are any cans of peas in the pantry...I'm hungry.