With my first, I was in school and BF, and I got back to pre-pregnancy weight but was never 'thin'. After ODD, I got back to my normal size, which is 'trim' but not skinny. I found it actually harder to keep teh weight off once she weaned.
After DD2 was born, I lost the weight pretty easily, but kept an extra 10 pounds on me. Now I'm tandem nursing DD2 and DD3 and I've cleand up my diet a bit and I'm back to trim, and almost skinny.
For me, it was easy to blame the added weight on the fact that I was (or wasn't) breastfeeding. But really, if Im completely honest with myself, I kept extra weight on because I was very undisciplined in my eating. When I BF'd I used it as permission to eat a muffin or an extra slice of toast or a couple more cookies. When I got honest with myself about what I was eating, and got rid of the crap, the weight literally fell off effortlessly. BF has definitely helped me lose weight, as had returning to work - I don't have the access to food, or ingredients for baking...deadly! I bring a health morning and afternoon snack, and only allow myself plain coffee or tea (e.g. no lattes, pop, or hot chocolate) and I don't allow myself to go to the cafeteria - I only allow myself water once my snack (usually raw veggies) is gone.
I eat whatever I want on weekends, and I'm still losing weight. I don't feel deprived, and my skin is better having dropped most of the sugar and baking out of my diet

I'm kind of worried about weaning one of the girls, because I might have to be even more disciplined with my diet. That would be a bummer!
Edited to add: in my view, it's tough post-baby to lose weight because (a) most new moms are at home at least for a little while, with unlimited access to snacks; (b) most new moms are sleep-deprived to some extent, which makes their bodies crave fat and sugar; (c) most new moms are relying on more prepared foods at least in the short term because it's busy being a new mom and cooking from scratch is more challenging.
At least - that was my experience.