Answer: None. Cardio (stairmaster included) simply does not challenge your muscles enough to gain muscle. You will NOT gain muscle by doing the stairmaster.
And even strength training is not going to give you thick legs. It is very difficult to gain muscle while running a calorie deficit to lose weight, as the body tends to burn protein for energy rather than creating new muscle tissue. And without strength training, up to 25% of your weight loss can come from lost muscle, rather than fat. Given fat is considerably bulkier than muscle, (check www.dailyspark.com/blog.asp?post=quiz_how_
for a great photo of this), this will actually result in fewer lost inches than if you included strength training in your program.
Weight loss is an all-body process, and cardio (of any kind) will not reshape your body. But if you want a smaller and leaner body, you should include BOTH strength training and cardio in your overall program.