Increase the size of bootcamp of Windows 7 on virtual machine
You might be able to use the application GParted to do this. It's not an extremely simple process, since, "The problem is that if you resize your boot/system partition, you will be completely unable to boot without repairing windows," according to this guide from How-To Geek. However, that same guide will guide you through the entire process of doing so.

Mac Rumors has a list of several alternate methods to accomplish your goal.