this is described in the doc, but perhaps not easy to find ;)
You can do what you are looking for. It is controlled via the layout strategy for one, and then via the layout itself. The layout can describe individual layout JSPs (or Servlets) for each 'state'. The state is set by the layout strategy. The strategy is also the one that can move portlets into different regions depending on the state, etc. Read the layout and theme doc. It is all in there.
The default layout is using two JSPs, one for 'normal' and one for 'maximized' state. The Maximizing Strategy filters all but the portlets in the navigation and the maximized region, and temporarily moves the maximized portlet into the maximized region.
So I guess in your case you want to only change the strategy to only filter the not maximized portlets in the region that contains the maximized portlet, hece the maximized portlet gets the entire region for itself. The rest should already do what you need.