Beware Chris: it's worse than you think. If you don't check the right box (which can only be found by holding down the download button on iCloud.com) it will download everything in a compressed format for storage purposes. This means 9mpx HEIF photos reduced into 3mpx JPEGs.

I don't believe however that it does this if you request a copy of the data sent to Google Photos on the server side. You can download them from there at full res.

If you sub to MS365 they give you a Terabyte of storage on OneDrive. If you can get the originals on your Mac it's possible to sync them back to OneDrive from the Finder.