[Phil G4SPZ]

I am old enough to remember the days when a U2 cell had a zinc casing covered by a thin paper wrapper that could be peeled off, and when an Ever Ready 'Bijou' 3-volt pen torch battery could be split into its two 1.5 volt AA-sized cells... I did, at one time, think that cells had increased in diameter due to some form of leakproof wrapping, but I don't think this is the case.

A couple of days ago I peeled the plastic wrapper off a couple of modern alkaline D cells to enable them to be squeezed into a 1970s Ever Ready AD35 battery case. The plastic wrapping was very thin indeed, barely a few thou', and the cell itself is actually sealed at the bottom rather than at the top.

Returning to PP3s specifically, I think that the casing is made of metal rather than plastic, and some manufacturers use a thicker metal than others, which as the metal is 'rolled' to form the seal at the bottom causes the variation in overall length.