tmux + powerline, bottom is too big

I recently switched from screen to tmux/powerline/tmuxinator
As shown below, there is a big unused space between the window and status bar.

What could be causing it?

.tmux.conf

set -g prefix C-t
unbind C-b
bind C-t send-prefix

set -g status-keys emacs
setw -g mod-keys emacs

set -g status-position bottom

# status bar
set-option -g status-utf8 on
# status bar


# windows
bind-key C-t last-window

# focus on first window#
 select-window -t 0

source ~/.local/lib/python2.7/site-packages/powerline/bindings/tmux/powerline.conf

and tmuxinator

# ~/.tmuxinator/zibann.yml

name: zibann
root: ~/Documents/zibann/

# Optional tmux socket
# socket_name: foo

# Runs before everything. Use it to start daemons etc.
# pre: sudo /etc/rc.d/mysqld start

# Runs in each window and pane before window/pane specific commands. Useful for setting up interpreter versions.
# pre_window: rbenv shell 2.0.0-p247

# Pass command line options to tmux. Useful for specifying a different tmux.conf.
# tmux_options: -f ~/.tmux.mac.conf

# Change the command to call tmux.  This can be used by derivatives/wrappers like byobu.
# tmux_command: byobu

windows:
  - root: ls -l
  - emacs: workon zibann && cd momsite && emacs
  - cmd: workon zibann && cd momsite
  - ipdb: workon zibann && cd momsite && emacs
  - dbshell: workon zibann && cd momsite && python manage.py dbshell
  - logs:
      layout: main-vertical
      panes:
        - tail -f momsite/momsite/log/celeryd_error.log
        - tail -f momsite/momsite/log/celerybeat_error.log
        - tail -f momsite/momsite/log/uwsgi_out.log
        - tail -f /var/log/nginx/error.log
  - supervisor: workon zibann && cd momsite # && python manage.py supervisor --config-file=momsite/conf/supervisord.conf

C-b, D

and detached one of the clients and volla! (So there must be one and only one client attached at one time I guess)