I am guessing that your original png file doesn't have much meta data. But webp may be attaching some meta data to resulting image.
Also make user that they are not applying anti-aliasing and create shades of colors at the edges. PNG files can be amasingly tiny at high resolution thanks to limited color count in the image. Increasing image count can increase the file size without changing the resolution.
Rotating image can cause anti-aliasing like effect too.
Fosstodon is an English speaking Mastodon instance that is open to anyone who is interested in technology; particularly free & open source software.