Appropriate use of "reaching" when the subject gets smaller and not bigger

According to the Oxford Online English Dictionary, definition 2.1:

Attain or extend to (a specified point, level, or condition):

unemployment reached a peak in 1933

[NO OBJECT]: in its native habitat it will reach to about 6 m in height

Attain certainly can mean both directions, so I think it can technically be used in that fashion.

As far as should? Given that smaller is better in this case, I think it's reasonable. Certainly it is common to say:

I reached my goal weight of 140 lbs

which certainly wasn't reached from a lower weight in the majority of instances. For your particular usage, "shrinking" is probably more common (emphasizing the reduction in size), but I think "reach" is perfectly fine.

I would not use "reached" to mean regressed (ie, got worse).

Though the underlying word picture of reach is extending or stretching out, it has embraced metaphoric applications for a long time:

Old English ræcan, reccan "reach out, stretch out, extend, hold forth,"
also "succeed in touching, succeed in striking; address, speak to,"
also "offer, present, give, grant,"
from West Germanic *raikjan "stretch out the hand"
(cognates: Old Frisian reka, Middle Dutch reiken, Dutch reiken, Old High German and German reichen),
from Proto-Germanic *raikijanau,
perhaps from PIE root *reig- "to stretch out"
(cognates: Sanskrit rjyati "he stretches himself," riag "torture" (by racking);
Greek oregein "to reach, extend;" Lithuanian raižius "to stretch oneself;" Old Irish rigim "I stretch").

_{etymonline.com}

As other answers have indicated, the current definition seems to be quite consistent with attaining a goal or hitting a target--even if the target implies the restriction to a fractional proportion.

I'd go along with Joe's suggestion and use "sometimes shrinking to...". It follows naturally from "highly compressed", I'd say.