Why the resulting WebP images are bigger than the original PNG or JPG files?
The WebP compression algorithm was created, according to Google, as an alternative for both PNG and JPEG files. Generally, the resulting WebP image is on average 26% smaller than the original PNG, and 25-34% smaller than the original JPEG. However, in some particular cases, 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 sometimes, especially when the original file is already optimized. 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. In fact, our work is focused on our API, so when an image is sent for optimization, its WebP counterpart should almost always be smaller than the JPG/PNG/GIF. And if it isn't, just let us know!