Sure, it's possible. Site size is generally not a ranking factor. Not directly, at least. As Jim points out, the larger site may be more useful to the user, which would attract more backlinks, etc.

Bottom line: Focus on making your site the best. If that means adding more great content and making it bigger, great. If you can make it great without adding more pages, that's fine, too.

You should change the links on your website to link to the www version.

It's unlikely that updating your site to 301 redirect non-www URLs to www URLs would negatively impact your site. Google actually recommends that you do this: http://support.google.com/webmasters/bin/answer.py?hl=en&answer=93633