"Big" ones are growlers to  me, smaller 1l ones are either riggers or flagons.

I must be missing something. Can't you just buy bottled beer?

Jonski: There was (is?) a pub in London, can't remember the name, might have been the Frog & Firkin that sold Tshirts that said "Frog Od's Sake, Buy Me a Firkin Pint". 

Another one was "I've flounder great plaice for a firkin pint"

Geektastic:

 

I must be missing something. Can't you just buy bottled beer?

 

Possibly you are.

Yes you can just buy bottled beer.

Bottled beer is or used to be different to the draft beer served out of the tap at the pub. A flagon was filled from a tap either in the bar or in the bottle store and was the same beer you got when you had a pint poured in the bar.

We have a couple of Sprig & Fern growlers.  The phrase "Fill my growler for me, please" never gets old.