exported product images are way bigger than they should be

PNG is the problem. It's lossless, so a photo saved as PNG is always going to be huge. PNG is for logos, flat graphics and things that need transparency. For product photos export as JPG at around 70-80 quality, or WebP, and strip the metadata on export. A 1200px product shot should come out somewhere in the low hundreds of KB at most, often less. If your JPG was "not much better" it was probably exported at 100 quality, which roughly doubles the size for no visible gain. Also check what size the image actually displays at. No point shipping 1200px if the product grid shows it at 400.