My first instinct was that JPEG XL would produce bigger images, probably for better quality.

Why is this downvoted. If anything, this is ~infinitely more useful than the blog post, which is n=1 data. Now we have n=2 data points, ~infinite update.

For n=3: it haven't even ever once occurred to me that jpg xl results in larger images, or is for larger images. I've first seen it presented as a new, better jpg file format, and I haven't questioned this based on its name.