Exporting certain photos as JPEG results in way-too-large disk files
I have just noticed that certain images, when exported from Photos as JPEGs, are way too large on disk. This happens reliably with certain images, but does not happen with other roughly similar images taken at the same time with the same camera (in this case an iPhone 11 Pro). I am using Photos v 6.0 (321.0.110) on macOS Big Sur 11.1 on a 2019 16-inch MacBook Pro.
For example, I start with four images of a dog taken in the same session. I import them into Photos from the phone. Then I export them as (let's say) Medium quality JPEG, maximum dimension 600px. Three of the exported files will be between 100K and 150K on disk. The fourth one will be 2.4MB to 3.5MB. (!!!)
This is fully repeatable in that the same original photos will always cause the super-ginormous JPEG files to be created. Fiddling with the quality of the exported JPEG has minimal effect -- the super-ginormous JPEG files will be a teeny bit larger with higher quality, and a teeny bit smaller with lower quality, but still super-ginormous.
If I export one of these same problematic photos as a PNG, the PNG file will be a sensible size (350-450K). So the problem only happens when exporting as JPEG.
If I import the original photo into Pixelmator and export as JPEG, the resulting file will be a sensible size. So the problem only happens in Photos.
I'm not sure how long this has been going on. Maybe it's a Big Sur problem, or a macOS 11.1 problem, or maybe it's been happening for longer and I just didn't notice until now. (That last possibility seems unlikely though.)
Has anyone else seen anything like this, or know any way to avoid it?
MacBook Pro with Touch Bar