Compress WebP

You usually cannot tell by looking, and it matters. WebP has both modes in one format: lossless WebP is a PNG replacement, and lossy WebP is a JPG replacement that also keeps transparency. Compressing a lossless WebP gives you a large saving because nothing has been discarded yet. Compressing a lossy one is a second pass with the usual compounding problem. As a rule of thumb, a WebP saved from a website is lossy and one exported from a design tool is whatever the exporter chose.

Only if something needs to open the file that cannot read WebP, which is a real problem outside browsers. For size alone the answer is no: WebP is the more efficient format, so converting to JPG at matched quality gives you a larger file, not a smaller one. If the goal is compatibility, convert. If the goal is bytes, stay in WebP and lower the quality.

Because filling it in is usually not a small change. A product photo cut out from its background, a logo, an interface element with rounded corners: put any of those on a page as a JPG and a white or grey rectangle appears around it. WebP is the only widely supported format that gives you both aggressive compression and an alpha channel, which is the specific reason it replaced PNG for that kind of image rather than sitting alongside it.

For most work, yes. AVIF is meaningfully smaller at the same quality - often by a third - but it is far slower to encode, and this tool has to fetch a 3.5 MB encoder before it can produce one, because no browser will write AVIF itself. WebP encodes instantly, is read everywhere including by software written a decade ago, and is already a large improvement on JPEG. Use AVIF where the file is going somewhere modern and the bytes genuinely matter; use WebP for everything else.