Converting backwards is a compatibility decision
WebP is smaller and better supported on the web than it was, but "the web" is not everywhere. The reasons to go back to JPG are all about the destination:
- An upload form that validates the file type and rejects WebP
- A desktop image editor from before 2020
- A print shop, a marketplace listing or a document template
- An email client, where support is still inconsistent
None of these are about quality. If your file is staying on a website, converting away from WebP makes it larger for no benefit.
Transparency becomes white
JPG cannot store an alpha channel. Every transparent pixel needs a colour, and this page uses white because that is what people expect. If the image is a logo destined for a dark background, the result will have a white box around it — WebP to PNG keeps the transparency instead.
Generational loss, again
Your WebP was probably already lossy. Re-encoding as JPG adds another round of loss on top. It is small at high quality, but it is one-way — keep the WebP if you might need to convert again later.
Questions
Why would I convert away from WebP?+
Compatibility. Some desktop editors, older office software, a few email clients and various upload forms still reject WebP. JPG is the format that opens absolutely everywhere, which is occasionally worth more than the file size.
What happens to transparency?+
It is filled with white, because JPG has no alpha channel and something has to go behind the transparent pixels. If your image is meant to sit on a dark or coloured background, that white area will be visible — convert to PNG instead.
Does the file get bigger?+
Usually yes, by roughly 30–50%, because you are moving to a less efficient format. That is the price of compatibility and there is no way around it.