Your iCloud Photo Library is storing the original image files, but the Photos Library on your Mac does store additional support files for browsing and sharing, for example the faces thumbnails, thumbnails of all photos, previews of the edited versions to be shared with the Media browser. It is to be expected that the library in iCloud will be significantly smaller.
For example, my library in iCloud is currently showing 131 GB, and the library locally on my Mac has a size of 173 GB.
But check, if you have any items in your library that cannot upload to iCloud at all. It could be items in an unsupported format (PICT files, or videos with an unsupported codec). And if you imported photos as referenced, they will also not upload to iCloud.
You can test this with a smart album:
File > New Smart Album
and add a constraint: Any item
- Photo is referenced
- Photo is unable to upload to iCloud Photo Library.