Picture Uploads from iPhone to Dropbox (file size varies)

Thanks for the info. This means you’re storing photos on your phone in the high efficiency HEIC format.

Just so you know, while the way an iPhone saves and exports its pictures isn’t obvious in terms of its internal process, I can imagine it goes like the following:

• The picture is taken via the camera app.
• The iOS device keeps the original quality on the phone in HEIC format, which uses the least amount of disk space.
• Dropbox makes the conversion to JPG when using the ‘Save HEIC photos as’ when using the automatic camera upload.
• iOS will convert the image itself to JPG when using the ‘Transfer to Mac or PC’ option.
• Emailing the image directly from the phone, or uploading it manually, should also convert the image somehow through iOS, though this isn’t listed in their options.

Essentially, in all three cases, the HEIC file is compressed to a JPG image, which makes the filesize larger than a normal HEIC file. I can’t vouch for the quality of the compression differences between when the Dropbox mobile app does it, and the iOS ‘Transfer to Mac or PC’ option, hence the filesize differences as two different apps are making the changes.

As to how the image is added to Dropbox manually, or when emailed to you, and the image is the smallest size, I don’t think it’s HEIC, since it ends in a .JPG image. I honestly don’t know how it's done, so it might be worth checking with Apple, since this is directly through their own system.

While I don’t have all the answers for you (the adding of the image directly to Dropbox and emailing it to you is unknown to me), hopefully this should shed some light on the matter.