At a basic level, in the presence of insulin certain enzymes/hormones are activated which block the release of lipid molecules from an adipose cell.
The idea is that fat cannot be used for energy if lipids cannot leave the fat cell and be taken to another body cell for consumption. In the presence of insulin, the release of lipids from adipose cells into the bloodstream is strongly opposed. In other words, your body's cells have trouble accessing fat stores when insulin is up.
Within that framework, I don't really see how a drug which improves insulin sensitivity is going to rapidly make obese people thinner if they keep stuffing their faces, at least not in the long term.