PR = unanimous constituencies, not geographical representation

FPTP with its single-member districts is the minimum possible district size - 

yes when people say FPTP achieves local representation, they mean  the smallest possible local-ness, 

at the expense of fairness,

and of course under other levels of government,  larger districts (or smaller districts) are the "smallest possible size."

under FPTP, size of district is derived by the electorate divided by number of seats, into equal-sized groups of people,

Of course geographic size varies widely, thus making false FPTP's claims of local-ness -- a bit tainted for the massive districts in rural parts of the country , and rendered ridiculous when a city is divided into 8 or 20 "local" districts -

wherein each and every voter is said to be represented by the same member, as if.

while under PR  you get away from geographic representation, 

instead you have unanimous constituencies -- each unanimous constituency represented by its own elected member 

with a constituency concentrated in a particular place or scattered about, just denpding on where votes of that constituency find themselves.

DISTRICTING UNDER PR

unless you use at-large districting, which is not at all common, 

you have either:

- districts of different District Magnitude - different numbers of seats and voters in districts (but of course at about same pop.- or voter- per seat) ratio)

OR

-districts of same DM (Malta), with about same numbers of voters in districts (and at about same pop.- or voter- per seat).