Image size is increased when converted from jpg to webp with quality value 100

Including the reference to the Google documentation who came with the WebP format about the possibility of the file size getting increased when converted into the WebP format.

Yes, usually when converting from a lossy format to WebP lossless or vice versa. This is mainly due to the colorspace difference (YUV420 vs ARGB) and the conversion between these.

https://developers.google.com/speed/webp/faq#can_a_webp_image_grow_larger_than_its_source_image

The problem you face is that with JPEG there is are large number of variables you can manipulate to get different compression. That is the benefit you get from lossy compression. Lossless compression tends to have few (if any options). In lossless compression, the tradeoff is time vs. compression. In lossy it is quality vs. compression.

You are running a lossy compressed image through a second lossy compression process and getting rather unpredictable results.

The real comparison would be to take your PNG images and compress them using webp and JPEG using various settings to see what quality you get compared to the compression.