It's a big security hole if you don't set your feeds to "Summary"
Take a look at your current WP feed settings. Under Settings > Reading, I recommend that "For each article in a feed, show" should be set to "Summary." Current versions of WP install with this set to "Full text" by default so you need to change it.
(Incidentally, this should register as a warning message at the top of the plugin's admin panel unless you have checked the plugin option "Ignore warning messages." It's also covered under "Locking down your site" in the installation instructions: http://wordpress.org/extend/plugins/wp-members/installation/ )
You can also gain a little more control over your feed excerpts leveraging some built-in WP functions. Here are a couple examples:
// set the number to whatever number of
// words you want the excerpt to be.
$content = the_content;
$len = strpos($content, 'more');
$len = $len-10;
Hope that helps.