The Info in Photos is showing you the current size of the photo, which is not necessarily the size of the original image file. When you edit a photo and add adjustments to it, the size may change. The Info in Photos will show you the size of a high quality preview of the edited version with the adjustments applied, usually a JPEG. When you export the edited version as a TIFF the size will be much larger.
The size shown in Photos does not depend on the download from iCloud. Photos will show the same size in the Info whether the original has been downloaded from iCloud or not. But if you want to be able to make backups of all your photos on your Mac, ensure that all originals will be downloaded from iCloud, using the setting pointed out by bbesspete. Only then the Time Machine backup of your Photos Library will be complete and contain all high resolution originals.
But even after we enable "Download Originals" we cannot be sure, that the originals have been downloaded, because there is no status message that tells us, that the download has been completed. I am occasionally copying my Photos Library to an external drive to have a backup copy. When I then open the copy in Photos and get a warning about "incomplete items", I will know, that some originals do still downloading.
