Maybe your other pictures are in another Library. On your Mac, does Photos>Settings>General look like this?

In particular, is "Use as System Photo Library" grayed out? "Grayed out" means that this is, indeed the System Library. It's only the System Library that syncs with iCloud Photos, so if it's too grayed out, then iCloud is using a different Library.
To see other Libraries that might be on your Mac, you should close Photos, and option-click on the Photos icon. Then you will get a dialog that shows all the Libraries that Photos knows about.

I can see that my Favorites Library is the one that is the System Library, but the Nikon Library is the last one I opened.
Does Photos>Settings>iCloud look like this?

I have "Download original" checked, but you may have "Optimize" checked.
These are the first things to look for. Let us know what you find...