The 1400 bytes and 10% thresholds came from WPT. #1513 (comment)

The combination of these two served to exclude suggesting that already compressed resources (like image formats such as PNG) should be compressed, since they can just happen to maybe be a couple bytes smaller given extreme cases (like a small image data section and tons of metadata, which typically isn't compressed).

But using a percentage seems wrong to me, since saving 1MB from a 11MB resource (<10%) is something that seems obviously good to do.

So here I'm suggesting only considering the % threshold if the total savings is less than 20kb.