1500x1500 is not arbitrary. Google Merchant Center asks for at least 500x500 and recommends 1500x1500 or above, because that resolution holds up on high-density phone screens, larger Shopping surfaces and TV placements without being resampled by the platform. It also sits comfortably under the 64-megapixel ceiling, which a 6000px camera original does not. The trap is doing this badly: upscaling an 800px web image to 1500px adds pixels and no detail, and Google explicitly says not to scale up or submit thumbnails. You want to start from the largest original you have, crop to the product, resize down to 1500x1500, then compress — in that order, because resizing first does most of the size reduction and the quality slider only has to shave the remainder. Skwish does the resize and the compression in the browser with a live per-file readout and a batch queue, so a whole catalog is one pass and one ZIP. Nothing leaves your device.