WebP file size increased by 20-60%, compared to Pillow 7.1.2

Load the image linked below, convert it to WebP.

Same file size as with Pillow 7.1.2 (± 5%)

File size increased by almost 50%, compared to Pillow 7.1.2.

from PIL import Image
source = Image.open('source.jpg')
source.save('dest.webp', quality=80)

Testing, I find this is the result of #4547

If I pass in method=4 to the save method -

from PIL import Image
source = Image.open('source.jpg')
source.save('dest.webp', quality=80, method=4)

Then I get the same file size as before the change.

So the different because we changed the default behaviour. I've created #5450 to resolve this by changing our default to 4 as well.

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