shrinking Betelgeuse

I read an article today in a non-scientific but serious magazine
(actually its online version) that Betelgeuse has shrunk by 15% in the
last 20 years or so (whether radius or volume or angular size isn't
clear, but in any case that's appreciable). I've known for a long time
that Betelgeuse is probably quite close to becoming a supernova, but of
course "quite close" in astronomical terms could be a rather, errm,
astronomical time for a human. I doubt one could even have said
hundreds of years or thousands or even tens of thousands (but probably
less than a million).

Does the measured shrinkage indicate that the time until supernova might
be relatively short even on a human timescale? If so, what are the
chances of it happening within, say, 30 years? Is such a shrinkage
expected? Apparently the luminosity hasn't changed; is this expected or
a surprise? (If the latter, then all other estimates are probably
rather uncertain.)

A web search for +betelgeuse +townes gives 1.860 hits, the first of
which is http://www.astronomy.com/asy/default.aspx?c=a&id=8342 which is
essentially what I read (in German) today.

Is there anyone here familiar with the late stages of stellar evolution
who can fill in the details? Popular-press accounts are sometimes
distorted, while an interested non-expert with little time like myself
can't read all the relevant refereed-journal papers.