I think hormones play a role too. I took birth control after #1 and struggled with supply issues and never lost any weight. We EBF for a year.
I am now almost 3 months post partum with #2. I am EBF again, but this time no birth control as hubby is getting a vasectomy. My supply is great, I make an extra 3-4 oz a day and I'm pumping/ working full time. I have also lost 20 pounds extra from what I weighed when I got pregnant (granted, I was 50 lbs overweight before having both babies and didn't gain more than 15 lbs with each).
I am now on weight watchers as a nursing mom trying to lose the rest of the 30 lbs to get me back to my newlywed weight. So far I have lost and gained the same 4-5 lbs over 3 weeks. So we shall see if I have any other success.