Photos app reduces the size of my photos as I export them.

Nierninwa wrote: …I generally don't use the app on desktop anymore

That's certainly fine, but it sounds like maybe you haven't reached a full appreciation of what the Photos app can do. One thing you should know is that Photos stores picture information in a database, not in files. So the file you want (if it's not the unmodified original) does not exist until you export it. Then you have lots of choices.

the file size reduces from 11-12 MB to around 3-4 MB. 

I'm not sure what it is about file size that's important to you, since that can indicate a variety of things. But in Photos, you have lots of choices in the "quality" of an export. In the Export dialog you can choose jpeg

and then choose size, which controls the number of pixels in the output. For a jpeg you can also choose "Quality" which controls the degree of compression. Either of those changes the file size.

If you choose TIFF for your "Kind," then you can choose bit depth of the picture. The Compression for a TIFF file is lossless, so you will get a much larger number of megabytes, but you may not be able to tell the difference. I exported a 2.3MB iPhone image as a TIFF, and it became 50.7 MB. Of course, it didn't have any more information than the original.

If you choose HEIC you can get the same quality with a smaller file. I exported a 1.3MB HEIC as a jpeg and it became 5.6MB-- but, of course, it couldn't possibly have more information than the original.

As Yer_Man says, importing to Photos may not be the ideal way to batch change the dates. I use  GraphicConverter ($40)  which will deal with file attributes of any sort and is very useful for picture metadata.

Let us know if you'd like more information about Photos…

What do you get when you export from the library as edited jpegs and set the various settings as follows:

That always gives me more than the original jpeg file. Don't know how it will work on your files but may be a workaround.

OR try holding down the Option key when dragging the thumbnail to the desktop. That gives my the original file even after I've edited it considerably, i.e. cropped.

What export settings are you using?

But more importantly, why use a database app like Photos for a task like this. It is designed to preserve the original and you don't want to do that. An app like A Better Finder Rename is much more the kind of thing you need, or Graphic Converter or any of a dozen other apps which are not database driven.

Thought somehow when I selected "maximum" quality, original colour profile and full size they're somehow… bigger files than I started with?

It's about compression. Jpeg is a compression format not an image one. When you take a photo your camera (or phone or whatever) creates a raw file from the data on the sensor, then processes it into a photograph, and compresses that photograph into a Jpeg.

When you view a photograph stored in a Jpeg, what actually happens is the file is decompressed to show you the image.

When you export from Photos the app creates a new file and puts that photograph into the file, and, as it's a jpeg, compresses it. And the file can be larger at maximum setting as the app is applying less compression than the camera did.

Nierninwa wrote: … they're somehow… bigger files than I started with? I am confused as to how Photos does things but oh well.

When you edit a picture it adds information.