Image Capture doesn't use full-resolution photo from iPhone

Hi,

I recently realized that my iPhone had been configured to save photos to iCloud, and I didn't realize this until my iCloud storage got filled up, so I used Image Capture to import all of the photos and movies from my iPhone to my Mac (running Catalina). After doing this, and verifying that the image copies looked good, I did the "Disable and Delete" on my iPhone, which pretty much emptied my iCloud storage. I then received an email stating that:

"iCloud Photos has been disabled on all your devices and new photos and videos are no longer uploading to iCloud."

which I expected, followed by:

"Original format, full resolution versions of your photos and videos are not stored on your devices. You must download your full resolution photos and videos to at least one device before they are deleted from iCloud Photos in 30 days."

which surprised me, because I didn't think that I had specified "optimized storage" for my iPhone. But to be safe, I went to icloud.com, selected everything in the "Photos" section and downloaded them to my Mac. After the download completed, I decided to compare the file sizes of some of the photos between the versions transferred via Image Capture and the iCloud website download, and to my surprise, some of the photos transferred via Image Capture had a smaller file size. I hadn't expected this, but is this the way that Image Capture works, that it or the iPhone isn't "smart" enough to access the original resolution version of a photo? Or might there have been an option I'm not aware of that would have allowed me to specifically request a full-resolution version of each photo to be retrieved prior to the transfer to my Mac, if this needed to be done?

Thanks,

Ken