You may want to use something other than Word to generate your intermediate html, because Word is known to insert a LOT of extra tags and redundant full font instructions, causing considerable bloat to your final epub files.
For example: instead of just using <p> for each paragraph (linebreak in word) it uses <p fontface=blabla fontsize=blablabla fontcolor=blablabla> even though that's completely uneccessary, unless you used a different font specification in between.
That and other things add up sufficiently to potentially double your epub file sizes.
Opening your doc file in Open Office and saving as html makes cleaner html. Also, Open Office has a plugin available to save files directly to epub.
There's other ways to convert an office doc file to html, but I don't know what's available on the Mac