You have client windows. Your main client window swap, should always be the same resolution in the foreground (I.e. the window you drive from) The Background windows (Depending on what you set it up as) "CAN" be the same size as your main client window, but typically are not, as its a waste of resources.

If you swap from client window to a slave, the slave window should come up to whatever preset resolution settings you set up in wow, or unless you made a specific setting alteration within ISB... which im not really sure where that is... in fact.