File sizes can be reduced by using more processor intensive compression. 
Compression programs frequently offer different levels and OOo only uses a
medium setting.  Large savings can be had by recompressing OpenDocument files on
higher compression levels and this could be implemented into OOo to give users a
way of reducing file sizes to make them more suitable for Internet use.

Similarly, images in files can be reduced in size by using programs like
jpegoptim and optipng (both open source, there are also many others) to greatly
reduce the file size again.  Options like JPEG quality could be offered in OOo
so that users could reduce their file sizes easily.

A combined set of options could be offered for users to select compression
levels, quality factors etc. for their files.  Some of these options might
require work on some of the compression and graphics libraries behind OOo as
there are more modern ones that offer better file sizes/performance.  For
instance 7zip can produce smaller ZIP files (no, not it's native 7z) than zlib
and is also very fast at doing it.