Why the resulting WebP images are bigger than the original PNG files?
WebP compression algorithm was created, according to Google, as an alternative for both PNG and JPEG files. While generally the resulting WebP image is, on average, 26% smaller than the original PNG and 25-34% smaller than the original JPEG, sometimes the resulting image can be bigger. Much bigger. We encountered cases when the WebP version of a PNG was double or three times the size of the original file size.
There is nothing to worry about, it is just a limitation of the WebP algorithm sometime, especially when the original PNG file is very well optimzed. Google explains it on this section of the previous page: Can a WebP image grow larger than its source image?
Here at ShortPixel we are always searching for ways to make the WebP files smaller but we’ll implement them only if the images’ final quality won’t be affected.