The percentage how much smaller WebP images can be compared to JPG and PNG highly depends on your individual case. E. g. what type of images do you have, how much are your JPG/PNG files already optimized through lossy/lossless compression techniques, what are your quality expectations/requirements...
According to Google following file size reductions can be realized with WebP :
WebP lossless images are 26% smaller in size compared to PNGs. WebP lossy images are 25-34% smaller than comparable JPEG images at equivalent SSIM quality index.
and
Lossless WebP supports transparency (also known as alpha channel) at a cost of just 22% additional bytes. For cases when lossy RGB compression is acceptable, lossy WebP also supports transparency, typically providing 3× smaller file sizes compared to PNG. (source)
However, with our own study based on Magento Luma sample images, we found that much higher savings and reduction of file sizes up to 75% and more can be realized while still having decent image quality - see chart and table below.
Please see here for more details and a (high-resolution) comparison between JPG and WebP in different quality / compression levels.
Image FileFilesize (MB)Saving (%) Original .jpg71  WebP Quality 905917 % WebP Quality 752664 % WebP Quality 501776 %