What are the advantages here? I'm still a newbie to this and was wondering what the bigger pulleys where for. And are the cams the deformed circles? Could someone explain this to me?

Depends if you are talking about a single cam or dual cam bow. Your single cam bows will have a cam on bottom and roller on top. Most modern bows your essentially the same size roller up top. Older technology used smaller rollers as the technology wasn't there to take advantage of the larger size. Not sure if that was more riser technology cam technology.

As far as cam shape, the more round your cam is the smoother the draw. However this gives up speed. The more rectangular often the harsher but also faster.

Ok, so a Roller is a perfect circle no matter the size?

One a single cam yes for at least all of the modern bows I've seen. When they start changing shape they become dual cam bows. Or hybrid cam, cam and 1/2 etc

Here is a very good guide on the various aspects of a bow.
http://www.huntersfriend.com/bowselection.htm