When an image looks softer after conversion, it is tempting to blame the new file extension. In practice, several different processes can reduce apparent sharpness.
Resizing to fewer pixels
If a 3000-pixel-wide image is reduced to 800 pixels, fine detail must be discarded. The smaller image may be perfectly appropriate for the web but cannot preserve all original detail.
Upscaling
Displaying a small image larger than its native dimensions requires interpolation. New pixels are estimated from existing ones, which can make edges appear soft.
Lossy compression
JPEG, lossy WebP and lossy AVIF can remove detail to reduce file size. Aggressive settings may smear texture or introduce ringing around high-contrast edges.
Repeated conversion
Repeatedly encoding from one lossy file to another can compound artifacts. Keep a high-quality source and create final delivery files from it whenever possible.
Browser scaling
Even a good source can look soft if CSS displays it at an awkward fractional size. Compare the actual pixel dimensions with the displayed dimensions before deciding the converter is at fault.