The 1.4 MB shown in the Info is the size of the HEIC still frame. A HEIC file has a better compression than a JPEG of the same quality. So the size is only half of the size of the JPEG version. If you want to save storage, keep the images as HEIC files and discard the .MOV, if you do not need the LIVE effect.
When you export the current version instead of the original, Photos will render a new image file, depending on the settings for the JPEG quality. If you use "Original Size" with "JPEG Quality: Maximum" the file may have a larger size than the original, because "Maximum" will use nearly no compression at all. Usually "Jpeg Quality: High" will give you an image file of roughly the same size as the original JPEG, but still larger than the HEIC will have been.