the problem is that the content of a window is different at different window sizes. there is nothing to zoom inside, but rather to rearrange. just imagine an explorer window, it will show more icons if it gets bigger.
the ElementaryOS window manager has such a feature. you can see it in the first video here: http://elementaryos.org/journal/meet-gala-window-manager the developers there have chosen to actually zoom the content during the animation.
looks pretty good actually. but imagine there would be a user option for slower animation speed ... the result would just suck.