I thought you were writing in because you were genuinely puzzled, not because you were going to go give your wife a lecture about diet. and exercise. She will only feel like you are being critical if you do. To show you that you are on the same team as she is in taking care of this family, ask her to show you how to make the dinner (and do it!) Do the dishes, change the baby, take the baby for a ride in the stroller, feed the other kids if there are some, and pick up things around the house. She will love you for it.
Hey, no offense but I think I'd lose my cool if my husband was asking how I could lose weight after birth. lol. Wowie, don't bring that up to her or be one of those men that pick on his wife's weight. We gain weight after we have a baby because we are exhausted, not sleeping well and find less time to take care of ourselves. It's the plague of women. We give our men a child and become worker bees for the next 18 years to the kids, house, our jobs, etc. The best thing you can do for your wife is pick up the duster, make dinner and do some laundry. good luck
Sometimes all the sitting with a new baby is an unnoticeable change compared to the routine before the baby came. I gained 15 lbs. when I was nursing, and realized it was because I clocked so much time on the couch, holding and nursing the baby, that I was never spending on the couch for long stretches of time before.