File size is not the same as image size. Image size is counted in pixels, the width and the height of the picture; file size is the space the file takes on disk, and it depends on the pixel count, the format and how strongly the file is compressed. Two images of identical dimensions can differ in weight many times over.
An image above a marketplace's limit is rejected at upload or fails to load from a product feed. The usual remedy is to export the same pixels as a JPEG at a slightly lower quality setting, which cuts the weight sharply before any loss becomes visible. Reducing the pixel dimensions is the last resort, because it can take an image below the size that zoom needs.