Just how many people do you suppose have died while laying on a scale senstive enough to make this measurement? How many people have ever measured their weight to this accuracy, and know it to that accuracy at the moment of death so as to be able to detect this change in weight?
I think the answer to both these question is exacly 0.
Why do you think anything that is said in a movie is in the least bit crediable?
The soul is metaphysical, it is not of the world of measurment. I as a physisist am only able to deal with measurments, with and must have measurements in order to understand the PHYSICAL universe. This is a fundamental restriction which PHYSICS will never overcome. The similarity of the emphasised words in not cooincidental. Physics is the study of the physical universe. The soul, or essence of life, is not measurable, there for not a part of physics.
I suppose when you die, your lungs collapse. This could account for a weight loss as air is expelled from the lungs.
I personally don't believe in God, or a soul. I believe that when your gone, that's it. You've had your chips.