No, they are the result of calculations, not estimates. First of all, they represent an upper bound on the mass loss (not change in size) a star can undergo during the "life" of a star. That calculation comes from the maximum energy conversion available via the various nuclear reactions that take place in a star such as the sun, and would only be that large if the entire supply of hydrogen in the sun were fused into heavier nuclei, which can't happen because most of the hydrogen in the star lies in regions outside of the fusion reaction zone. Most stars switch to helium burning after about 10% of the hydrogen is fused, which for the sun should be in about another 5 billion years.
The point is that the mass change of a star such as the sun is exquisitely small, and hence the change in gravitation is also minute.
What is difficult to understand is that if you are interested enough in the subject to have formulated "your own hypothesis" that stars are burning fuel like giant furnaces, why didn't you do some simple research on the subject on the internet to discover that hypothesis could not possibly be true? The text book I cited for you gives a wonderful discussion of the arguments that refute all sources of energy in stars (including chemical reactions, as you proposed) as being adequate other than fusion reactions.
Thanks for your link. The topic is of interest and I'll save it for future reference.