Not delete, Just discard the pages of no importance. It's their algorithm, nothing can be done, just optimize your pages and gain links in support to them.
we need to call to Sergey Brin and ask about it P.S My site is internet store, it is very bad for us.
there is a set of formula they follow for indexing n nobody knows.. the index decreases as google crawls and finds details about the website.. removes duplicate and also spammed links to the original site..