Try to set the size of the figure (in inches) before you save it. You can do this when you initialize the figure by doing:
figure(figsize=(11.69,8.27)) # for landscape
or if the figure exists:
f = gcf() # f = figure(n) if you know the figure number
f.set_size_inches(11.69,8.27)
or in advance for all plots, using
rc('figure', figsize=(11.69,8.27))