[yummyfajitas]

It's also not very hard to lose 5lb if you are morbidly obese.

Harris-Benedict predicts that an individual should have an equilibrium bodyweight depending on their activity level and mean caloric intake. When your actual weight deviates from equilibrium, you will approach it along an exponential curve.

This means that if you alter your diet, your initial loss (or gain) will be rapid, while later losses/gains will be very slow. Graphs here:

http://www.chrisstucchio.com/blog/2011/weight_stability.html