Jeff, when I took a masters in nutrition, I was taught that weight gain or
loss was, indeed, a matter of calories in and out, that 3500 calories was
about equal to a pound.
However, I have since come to understand this is not necessarily true. There are so many factors that deal with the issue of weight gain and loss that simple caloric measurements do not tell the entire story. Some researchers believe that there is an actual virus that some obese people may have that keeps them from losing, regardless of their caloric defecits. I know, it seems impossible, almost as if it violates the laws of thermodynamics, but the viruses may not be aware of these laws. Other researchers have pretty solid evidence for different genotypes where calories are used more or less efficiently depending on distant past genetics. If your ancestors from 1.5 million years ago were from the African velt where famines are not uncommon, you may have in herited thrifty genes that very efficiently store calories, and give them up without no little fight.
I've known people who say they can eat a 3 ounce piece of cake and gain a pound. This seems impossible, unless they are drinking 13 ounces of water, but if the virus or genotype theories are correct, it may be that the body can take what seems like only a few calories and parlay them into fat at a rate very different from the standard accepted figgure of 3500/pound.