That is odd. Have you cleared your browser cache and cookies? I’m wondering if you have a viewing preference set that is associated with www.mydomain.com; the same text size preference may not be associated with the "mydomain.com" name.
That’s the easiest explanation I can think of. Maybe try viewing the 2 variations on a completely fresh machine and see if you can verify.
You could add a rewrite rule to your .htaccess file to enforce all "mydomain.com" to roll over to "www.mydomain.com"... something like this:
RewriteCond %{HTTP_HOST} .
RewriteCond %{HTTP_HOST} !^www\.mydomain\.com [NC]
RewriteRule (.*)
http://www.mydomain.com/$1 [R=301,L]
Even if it is a simple browser preference thing, putting in the rewrite rule will keep your pages all using the same url.