"Your weight is determined"
No. If at all, they are only a part of the problem and you have the chicken - egg problem. Do you have different bacteria because you are fat or are you fat because you have different bacteria?
Causes are:
-genetic
-epigenetic
-life style
-nutrition (cabrs!)
-viruses (no kidding, obesity may be infectious)
etc.
I think the jury is still out on that. Metagenomic studies have only become possible in the past decade, and consequently there haven't been any long tern studies in humans. Studies in mice have show that the normal microbiota is rather static throughout the full life of the host. If this extends to humans, then the microbes are the "egg" in the equation.