You can grab the axes handle with something like
Now, when you finish plotting on the axes with the handle h, you can run
set(h,'position',[0 0 1 1])
This will expand your plottable area to fill the figure. However, it may also cut off any title or axis labels you have, so you need to plan accordingly.
set(h,'position',[.05 .05 .9 .9])
will allow you to see more of the text outside of the plottable area.
Note that you can also use dot operator to set axes properties, but if you're running on older versions of MATLAB you'll need to use the get() and set() functions to modify properties.