There are a few things that can be causing this.
First, the Drop Shadow Boxes plugin adds 10px padding to the box. I intend to make this configurable but for now, you can remove it by adding this to your styles.css file in your theme folder:
Second, themes sometimes add margins to tables so you may want to check that out.
These two causes you can do something about relatively easily. Although if you're comfortable editing css and html you may want to implement the shadows manually yourself rather than using this plugin.
The third cause however is a bit more tricky. WordPress automatically adds line breaks here and there (
tags) - sometimes even where there is no line break in the content editor. I suspect this is what's causing most of the space in your case. There's nothing the plugin can do to prevent WordPress from inserting these tags inside the box I'm afraid. There are some plugins (like this one) that will disable the auto-formatting for you completely but I'd advise caution here because they can cause unexpected side-effects.
If you'd like to discuss this further, please get in touch via the support form.