Diet-Now,
I'm not on HRT, but I have been on the pill, and on tamoxifen and raloxifene. Birth control pills contain estrogen, which can make you gain weight. In my case, I gained around my abdomen.
Tamoxifen and raloxifene block estrogen from entering estrogen receptors in breast cancer cells, but in other ways they behave LIKE an estrogen (for example combatting osteoperosis). When I was diagnosed with breast cancer, another survivor I knew told me that tamoxifen "makes your stomach big," and, indeed, when I was on it I gained a bit more around my middle. Now I'm on raloxifene, and I have lost very little weight on Atkins (I also haven't been completely compliant). However, my stepmother has been on tamoxifen for nearly five years, and has remained slim -- this is because she exercizes. She runs several miles a day or plays tennis. She did tell me that on tamoxifen she is more prone to gaining weight if she overeats, especially carbs.