Having a lot of muscle will increase your base metabolic rate. Being a serious powerlifter, I imagine the increase due to muscle was significant (speaking just as someone who reads into this, not as a qualified professional!). Also, being a powerlifter, if you don't maintain the muscles, they would atrophy, taking a lot of weight with them.
Interestingly enough, I stopped powerlifting about a year ago and the atrophy hasn't happened as quickly or as much as I thought it would. Now, I say that with the caveat that I have NO idea how big my muscles were when I was at my strongest (which is also when I was 300 pounds), but I still have 25+ pounds of muscle that I didn't have the last time I was the size I am now (scientifically deduced from being able to fit into the same pair of pants that I did when I was 180lbs, when I am now 205lbs). I currently have a knee injury that's keeping me from doing any lower body lifts, and am only doing upper body bodybuilding style stuff twice a week. I'm absolutely not as strong as I used to be, but I'm still muscular... although, again, don't know how much of that is a hold-over from my powerlifting heyday because at the time I was like a muscular manatee... couldn't see the muscles for all the blubber on top.
The last point is important. Many people claim that muscle turns into fat if not exercised for maintanence. This obviously isn't true. What is interesting is where that idea may come from: the habit of eating a larger amount of calories to power a huge masses of muscle that doesn't entirely go away along with the muscle without conscious planning.
