Tweeted By @jeremyphoward
Turns out opencv by default uses RLE compression only, and that's the reason for the difference.
— Jeremy Howard (@jeremyphoward) October 16, 2019
PIL doesn't seem to support that (I couldn't see it it in docs, anyway), so you can't get quite the same mix of speed and size as opencv
h/t @eiopa for putting me on the right track https://t.co/WYq9HXKN66