With the combined strength of the Prussians and Franco-Bavarians, why not conquer the whole of Austria? What about Silesia was so important that it was all Federick II wanted of Austria, when he could have taken more?
The main reason that the Prussians declared on Austria was due to the fact that they wanted to increase their strength and did not endorse the Pragmatic Succession. The Pragmatic Succession was a treaty written by the old Austrian king stating that the HRE(holy roman empire) could be inherited by females.
The reason they only conquered Silesia is because they were economically very strong, and if they were in a position to, they would have taken more. the problem was that Prussia was not a very large nation during the Austrian Succession. the land they took in the war increased their population by ~50%. if Feredrick took much more then the Prussians would not have been able to effectively administer the land, or suppress rebellions.