How to set font size of Matplotlib axis Legend?

I have a code like this:

import matplotlib.pyplot as plt
from matplotlib.pyplot import *
from matplotlib.font_manager import FontProperties

fontP = FontProperties()
fontP.set_size('xx-small')
fig=plt.figure()
ax1=fig.add_subplot(111)
plot([1,2,3], label="test1")
ax1.legend(loc=0, ncol=1, bbox_to_anchor=(0, 0, 1, 1),
           prop = fontP,fancybox=True,shadow=False,title='LEGEND')
plt.show()

It can be seen in the plot that the setting in Fontsize does not affect the Legend Title font size.

How to set the font size of the legend title to a smaller size?