The server could just give the requesting device what it needs to display the content in both orientations. That doesn't necessitate also giving it the layout for 10 other possible screen sizes.
While most mobile devices have exactly 2 size, that may not be a good assumption. There are some devices that support split-screen and you would end up with more that just 2 sizes. You could send a list of all possible sizes, but If the split-screen is customizable it maybe not be feasible to enumerate all of them.
