If you feel like fiddling, you can:
a) uncompress the pdf with pdftk:
pdftk in.pdf output out.pdf uncompress
b) open the out.pdf with a text editor and modify the /MediaBox lines, this is the size of each page's "viewport", in points (1in=72pt, 1cm=28.3465pt).
c) compress again with pdftk:
pdftk out.pdf output final.pdf compress