Original plotting window does not initially respect rcParams value

Summary

I import matplotlib and then set the rcParams by mpl.rcParams.update( {'figure.figsize': (15, 9.3)}). When I attempt to plot the window, its size is plotted instead as [10.35, 9.3] and not what I originally specified. This has onlst started occuring since I upgraded to a larger monitor. It does not occur for all inputs, and the larger (32, 20) plots correctly the first time.

Details

I have previously used the following to ensure my figure-size in my plots is a consistent size:

import matplotlib as mpl
rc_fonts = {'figure.figsize': (15, 9.3)}
mpl.rcParams.update(rc_fonts)
import matplotlib.pylab as plt

However, I am now finding that for my usual default values (15, 9.3) this is being ignored. The following demonstrates this (This must be run in stages, see current output section):

import matplotlib as mpl
rc_fonts = {'figure.figsize': (15, 9.3)}
mpl.rcParams.update(rc_fonts)
import matplotlib.pylab as plt
# I draw some boring plot. 
plt.clf()
plt.plot(*[range(10)]*2)
print plt.gcf().get_size_inches()
print mpl.rcParams['figure.figsize']
plt.gcf().set_size_inches(15, 9.3, forward=True)
print plt.gcf().get_size_inches()

The initial plot size is [10.35, 9.3] and after it is [15, 9.3] as desired. If however I make the default very much large or smaller, e.g. (32, 19.3) then the figure window is correctly sized. I thought setting rcParams is the recommended route?

Details:

• Python 2.7.12 (inside a virtual environment, a must).
• Backend TkAgg (I want this kept as it is).
• Matplotlib version 2.1.0. (This bug/feature persists in version 2.1.2 also).

I have tried this inside and outside a virtual environment, and also tried Matplotlib versions 2.2 with python 3.5, and still it persists, and with all the backends I can muster (including Qt5Agg).

Possible known issue:

I have found the following issue 2716 which I think could be related, but there don't appear any fixes suitable for the rcParam settings route.

Current output:

Following the comments below is some example output (done using Python 3 to allow me to install the latest version of matplotlib):

Python 3.5.2 (default, Nov 23 2017, 16:37:01) 
[GCC 5.4.0 20160609] on linux
>>> 
... import matplotlib as mpl
... print(mpl.__version__)
... rc_fonts = {'figure.figsize': (15, 9.3)}
... mpl.rcParams.update(rc_fonts)
... import matplotlib.pylab as plt
... plt.plot(*[range(10)]*2)
... 
Backend Qt4Agg is interactive backend. Turning interactive mode on.
2.2.0rc1+124.gf1f83f6
>>> 
... print(plt.gcf().get_size_inches())
... print(mpl.rcParams['figure.figsize'])
... plt.gcf().set_size_inches(15, 9.3, forward=True)
... print(plt.gcf().get_size_inches())
... 
[ 10.35   9.3 ]
[15.0, 9.3]
[ 15.    9.3]

DEMONSTRATION