Nope - that sounds right to me. Your original file will probably be encoded with H264 which is much better at reducing file size. With PhotoJpeg, there is much less compression (and presumably artifacts) although there is a long thread somewhere about that!

You could reduce the quality setting of your PhotoJpegs a bit - I use 85 which seems to be acceptable at all the main agencies.

Steve

Remember that you have decoded the video stream to get it into your editing software (assuming you aren't just directly clipping the start and end points which might not need any re-coding). So you have already got rid of the H264 codec at that point. If you ever try to save the file directly from your editing program as an uncompressed file, you will see that it is many times as large as you are currently seeing. So you need to choose a codec to compress it again for uploading and many agencies seem to prefer PhotoJpeg that ends up with a larger file size than H264. So the comparison you need to make is whether your PhotoJpeg file is "better" than a file that uses H264 for the final output. The agencies seem to think so.

Hope that is clearer.

Steve

I'm shooting some videos with an Olympus OM-d E-M5 Mk II, with the best possible quality.

For each video I cut the parts I don't need and I encode it with Photo JPEG, but the result is a lot bigger than the original file...

For example:
Original file: 21 seconds - 191 MB
Exported file: 21 seconds - 464 MB

Am I doing something wrong?
Is there a lossless way to decrease the exported file size?

QuickTime, for example, lets you cut files without re-encoding. I would, however, recommend basic (color/contrast/exposure) editing to heavy editing so the clips look their best. As soon as you do that, you need to re-encode, and it's best to do that with a higher bitrate than the original to keep the quality.

Even though it's nice to have raw material as an editor, the vast majority of best-selling clips I've seen are quite heavily edited/graded.

This is an age old debate, but you can't improve the quality of the clip by re-encoding it as a Photo JPEG. I don't know what editor your processing your clips with, but I would suggest you look for solutions that use the native file format i.e. H264, and take care to setup the output bitrate settings to something comparable to the original file. This could be something between 75 and 100 Mbps at a variable bitrate.
You will find arguments to justify Photo JPEG for stock but in my experience, having uploaded both formats, for the time being H264 is the least problematic.

As soon as you make a change (color/contrast/exposure etc.) you're essentially unpacking and need to encode again. A higher bitrate than the original can then indeed mean better quality. That is, not better than the original, untouched file, but better quality than encoding with the same settings as the original.

Especially with heavy compression like h264.