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

[pix666]

What did you do?

Load the image linked below, convert it to WebP.

What did you expect to happen?

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

What actually happened?

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

File	Size
Source (jpg)	214 779
Pillow 7.1.2 (webp)	108 450
Pillow 7.2.0 (webp)	159 964

What are your OS, Python and Pillow versions?

• OS: Ubuntu 20.04
• Python: 3.8.2
• Pillow: 7.2.0

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

[radarhere]

Testing, I find this is the result of #4547

[radarhere]

Looking at the webp source code, the default 'method' value is 4 - https://chromium.googlesource.com/webm/libwebp/+/refs/heads/master/src/enc/config_enc.c#34

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.