A blog post by Mark Foo on Pickthebrain.com:

Being a big dreamer doesn’t mean that you walk around with your head in the clouds. It means that you’re seeking a purpose for your life and it means that you’re becoming fulfilled in the process.

People that get into the habit of dreaming big will accomplish those goals because they have the right mindset. You need to look at life with the perspective that you can achieve anything that’s possible. If you can accomplish big dreams, why wouldn’t you go after them? Why would you want to settle for small dreams?

See here a list of reasons why you should be a big dreamer.