Here's a link to the image

http://gallery.me.com/mick6425#100011

Hanging petunias tend to go brown and bare in the middle. look at where the flowers are they are at the end of the growing tips and that's the way petunias grow and flower. If you want to keep them full and bushy you have to trim back the longest branches so they make more branches and stay bushy. You also have to deadhead and that means pinching off the seed ball behind the flower

Are they getting a decent amount of sun? Lack of sun might make them go 'leggy' and not as full and bushy as well. Also, if you haven't been doing it, petunias are known as heavy feeders so in a pot they likely would benefit from regular fertilizing, else they kind of get 'scrappy' after a while.