Via Practical Tmux:
By default, all windows in a session are constrained to the size of the smallest client connected to that session, even if both clients are looking at different windows. It seems that in this particular case, Screen has the better default where a window is only constrained in size if a smaller client is actively looking at it. This behaviour can be fixed by setting tmux’s aggressive-resize option.
setw -g aggressive-resize on
Great if you are connected to a session on both a laptop and a desktop machine.