A fair format comparison changes one encoding choice at a time. Start from the original for every export; re-encoding a previously compressed file can add artifacts that obscure the result.
Use the same source image and target dimensions for every output.
Record the actual output MIME type and byte count, not only the selected extension.
Inspect fine texture, text, diagonal edges and flat color at normal viewing size.
Repeat with a second image whose texture and background differ.
Interpret the result carefully
A smaller file is not automatically a better file. A product label that becomes unreadable or a color that shifts in a way your storefront cannot tolerate may outweigh the byte reduction. PNG can be appropriate for crisp graphics, but it is not a promise of small photographs.