Cookie time

Rachael and I stopped off at this cookie place after lunch for a little dessert. I can never finish their cookies; they are simply too big. However, today, we had a very interesting sales clerk.

Me: I'll take an M & M cookie. I'd like the smallest one on the tray.
Clerk: They're all the same size.
Me: That's impossible.
a few seconds later
Me: That one's the biggest! I'll take that one (pointing to the smallest one on the tray.)
a few minutes later
Rachael: If they're all the same size, why do they charge by weight?