Then there is something in your site's CSS code that is affecting the cart layout -- since the cart works "inside" your page, it inherits CSS from your page. You would need to troubleshoot that yourself or contact the Webmaster who designed your site to figure out what's happening.
Handy tip: You can examine the HTML source of the cart itself by copying an Add to Cart button URL and pasting that URL into a new browser window/tab, then View Source on it just like any other Web page.