After reading through The Shadows of Night, DH remarked, "You seem to be ignoring the law of conservation of mass."
Well, yeah, I am. Shadows is a fantasy shapeshifter romance. Fantasy romance deals with magic or the paranormal (that which can't be explained by rational means). In other words, the laws of the universe as we know them don't have to apply. When writing fantasy romance, you can use pretty much whatever rules you want, provided the rules of your universe remain consistent. A person can become a butterfly, or a six-hundred pound tiger. Magic doesn't play by the rules of the real world.
Science fiction romance, however, is a different kettle of fish. I'm also working on a SF romance involving shapeshifters, and in that book the heroine can't change her mass. The laws of the natural universe always apply in SF romance. At least they should. One of my pet peeves with Star Trek: Deep Space Nine (geek alert!) was that the writers didn't always seem to grasp this concept. Odo, the shapeshifter, who normally had more or less the mass of a human, could become a drinking glass as light as any other glass, or a bird that could fly. It doesn't work that way in the real universe. Even if you could manage to cram a human-sized mass into the size of a glass, its weight would still remain constant. And something with the weight of a human isn't ableto fly on bird-sized wings.
One of my favorite series by Piers Anthony, the Apprentice Adept series (the first book of which is Split Infinity), perfectly illustrates the difference between fantasy and science fiction. Split Infinity and its sequels are set in two worlds separated by a "curtain"-- one world is ruled by science, and the other by magic. In the science world, Proton, shapeshifters have the same mass no matter their shape. In the magic world, Phaze, a woman can transform to a much larger unicorn or a much smaller hummingbird. I was always fascinated by this book, because it's both science fiction and fantasy!
But generally speaking, a romance can't be both. When you sit down to write this type of romance, it's important to ask yourself: Is this a fantasy, where magic applies, or is it science fiction, where the rules of the actual universe apply? If it's a fantasy romance, it must utilize internal logic, but doesn't have to abide by natural laws. If it's science fiction romance, it has to abide by both.