How can I reduce the file size of a photo in Photos app on iMac?

What is the file type of the photo? If it is JPG or HEIC, then it is already compressed. If it is a RAW photo, then you will need to Export it using one of those compression methods, then add it back to Photos. Delete the original RAW photo and you will be left with the compressed version.

With any photo, you can export it as JPG and adjust the compression level to reduce file size further and also picture quality, but HEIC does a good job by using the smallest file size while keeping the picture quality. There is no way to change the compression of a photo in the app and it requires that 3 step process to export, add back, and delete the original.

You can do this when you export it. If you export the photograph as a Jpeg you just export a lower quality version.

File -> Export.

Not sure where this is going…

You can use the built-in zip tool. Select the files in Finder, and select Compress from the menu. That’ll compress the photo. Probably.

But as mentioned above, common formats are already compressed. (I’ve met various cases where compressing poorly-compressible data results in more data; when the volume of data after compression gets larger, not smaller.)

If you are using Photos app and want to mail or message a photo to somebody, and the source photo resolution and size is higher than what the recipient tools can accept, you can ask that to export in smaller or lower-resolution formats.

Or if this is about storage usage more generally, if your iMac storage is constrained, compressing photos is unlikely to gain very much back.

Also consider an update to macOS 15.5, as that resolves various issues.