Because you're using multiple. Despite it still technically being a dropdown, it doesn't look or act like a standard dropdown. Rather, it populates a list box and lets them select multiple options.
Size determines how many options appear before they have to click down or up to see the other options.
I have a feeling what you want to achieve is only going to be possible with a JavaScript plugin.
Some examples:
jQuery multiselect drop down menu
http://labs.abeautifulsite.net/archived/jquery-multiSelect/demo/