You can use any windowing command in vimdiff, for example CTRL-w < to make the left window one column smaller. This can also be combined with a count, for example CTRL-W 5> to make the right window 5 columns smaller. Consult the help page with :help CTRL-w to get a list of window commands.
Additionally, you can use the mouse to change the size of the windows by clicking the boundary between both windows and dragging it into a direction. You might have to enable mouse support by issuing :set mouse=a first.