Results depend on text size

I'm just playing with these demos, so I don't know how much CSS needs to be managed by the developer, but the cosmetic quality of this form appears to largely depends on text size. When I choose a small text size, it looks good. When I choose a medium or large text size, text bursts out of the buttons, tabs, etc., even when I refresh the page. I tried this in both FireFox and IE.

Is this a bug and if so, is it one you're working to resolve?