I'm fairly light to begin with, but I've had some weight gain since my kidney transplant due to being on prednisone (oral steroid). I also wasn't able to ride much over the previous 4 years while on dialysis, so, I've been coming back since the spring. I had the kidney transplant a year ago, but you can't ride a bike for 2-3 months after, and by the time I could, it was winter. I ride for pleasure, but also for cardiovascular fitness, and to try to lose the extra 30 lbs I've gained (on a small 5'3" frame, that sure feels and looks like a lot). I've been riding (and also walking) quite a bit since the spring, but it hasn't made a dent in the extra weight. I don't think cycling alone is enough. It's great for cardiovascular and respiratory fitness, but it's not a big weight loser, at least not for me (except water loss due to sweating, which is only temporary). It's going to take patience and a long time of watching the fat and calorie intake on a daily basis, in addition to exercise. There's no way around it. To lose weight, you have to not take in as many calories as you are using. With cycling and most other exercises, you might gain some muscle weight, which is fine, but it doesn't necessarily follow that you lose fat weight, unless you limit and keep track of the calories. To be perfectly honest, sometimes when I come back from a ride, I'm so hungry that the calories I just used up, I put right back on and more.
So, I'm disappointed with the weight issue, but on the other hand, I've got to be happy with my fitness progress. A year ago, I could barely walk a couple of blocks, and now already, I'm riding at a steady, relatively fast pace for 2 hours at a time (no thanks to me, but all thanks to the person who died which allowed me to get a new kidney when my name came up on the waiting list).
Anyway, congratulations on resuming riding. It can be hard at first, and it takes some perseverance for sure. As for losing weight, your situation is a bit different than mine, but it takes a lot of patience.