Next time instead of saving as a .jpg try saving as a .tif and do not select compression. You'll see the file size is very different. JPG is a lossy compression - it loses information - creates artifacts etc! tif (tagged image format) is not a lossy compression, even when using the compression. A stock agency would NOT accept a jpg
Also if you open a jpg edit it and save it again and again it will keep losing more information as it recompresses each time. Of course they are greatly smaller files! A 5100 x 3400 pixel RGB file is about 49.6 MB when uncompressed! As previously mentioned this would be 11x17 at 300dpi.
A 35mm neg (about 0.9 x 1.4 inches)scanned at 4000 dpi would qualify being as this would yield 3600 x 5600 pixels prior to cropping!