Mel and Lydia are hardcore! Awesome photo!
Dianne, I wouldn't worry about your butt getting bigger. I had no fat on my butt to start with. Because I lost weight in my very large thighs it just gave my butt a larger appearance, as well as I've built up a little bit of muscle. If your butt is quite large I'd assume an amount of that is fat, with weight lifting you lose fat.
I think back to how I used to say 'well I want to lose some more weight first before weight training so I don't get bulky', I actually cringe now thinking about it. New Rules for Women addresses these myths for you. It will help you better understand your body and how it works. One of the myths is that people are worried about muscles growing bigger and pushing out the fat that exists. It's hard to build muscle, but you burn a lot of fat during weight training.
If you read the book, I'm sure your insecurities will go away. Mine did

It's changed the lives of a lot of women here.