Am I stating the obvious here but if "Airline A" leases aircraft from "Airline B" they may do it by means of a "Wet Lease, a "Damp Lease" or a "Dry Lease" or, indeed, "ACMI" of "Full Charter".
I've never heard of an "Accordian Fleet", might as well call it a "Trombone Fleet", it's equally as ridiculous, but I've leased in many an aircraft in my time, I recall one day when I leased in eight aircraft whilst my oppo was taking his 90 minute lunch break, did we have an "Accordian Fleet" that lunchtime?
Just because Airline A leases in an aircraft or few from Airlines B, C, D etc. doesn't neccessarily represent an increase in fleet size of Airline A, the aircraft still remain on the fleet(s) of Airlines B, C, D etc.