Low photo file size when exporting unmodified original from Photos app on Mac

However, I recently noticed that if I go to export and select highest quality instead, the exported file is about 10 times larger than the unmodified original version. Any idea what might be causing this? And does it necessarily indicate a different in photo quality?

An unmodified original is the image as downloaded from your camera. But to understand what's going on you need to understand that jpeg (or HEIC) are not image formats. They're actually compression formats. So what's happening is your camera takes an image, puts it in a file and the compresses the file. Result: A HEIC or Jpeg. When you view the file it is decompressed.

When you export a file though, what happens is Photos creates a new file and puts the image into it... and then compresses it into a HEIC or Jpeg. You get bigger file sizes because Photos is applying less compression than your camera did. It's the same image. It's not sharper, it's not more colourful or anything else, it's just the same image with less compression.

Where this might be useful is when you edit subsequently. The higher the quality the exported file (or the less compressed it is) then the more you will be able to edit it before you see a degradation of quality in the image.

You also might get larger files if you export high quality versions of HEIC originals. HEIC format is a more efficient compression process than jpg, so smaller files with higher quality.

For example I've just exported a max quality jpg of an HEIC File:

HEIC : 1.3MB

JPG : 5.3MB

16 Bit uncompressed TIF : 73.2 MB

The HEIC file will have higher quality (albeit imperceptibly) than the jpg - since the jpg will have been recompressed and some information lost.

So, on that basis, does it make sense for me to keeping storing my files in HEIC format rather than converting them to larger JPEG files?

I wouldn't make the decision on that basis. I would note, instead, that HEIC is a new format and not universally supported, though support is growing. There have been attempts to improve on JPEG before, the png format for instance, the JPEG2000 and so on. None have become universally supported.For instance, try find a camera that shoots in either of those formats.

JPEG, on the other hand, is universally supported. I'd base my decision on that.

In my opinion yes.

I always treat the original file like a digital negative - never to be changed or edited. I always use a non destructive editor (like the photos app, lightroom etc), or take a copy before editing.

It is relatively easy to convert to an alternate format as needed.

That does not make sense, I think we need more details, like how are you exporting?

If you are exporting an uncompressed Tif, you might see something like this.

Because the original has the best quality. Converting from one lossy format to another loses quality.