For virtually every PNG on my site, the original version is actually smaller than the Webflow generated version, and not by a small amount either.
Original 1730x1011: 111KB
Webflow 1600x936: 296KB (almost 3X!)
Webflow 1080x632: 152KB
Webflow 800x468: 104KB, finally a bit smaller, at less than half the resolution.
There's nothing very obvious going wrong with the variants, like a bigger colorspace being used than with the original, that I can spot.
I would strongly suggest, that if the original filesize is actually smaller than the responsive downscale, the srcset should exclude that variant.