The window fill feature is nice and gives xfwm4 some tiling wm properties. However I find myself often guessing how the window size might end up and quite often I don't like it, its too big or something else is wrong. In these cases I wish there was a feature to restore the window to its origin size. Currently there is none.
This has been discussed to death on the ML back in time and the answer is no, fill is a function (which works with a set of other parameters at a given time, like other windows positions), it is not a state that you can revert. Fullscreen, maximization, shade, iconic are states, "fill" is not a state.