Some of the photos files can be larger if they were Live Photos or HDR Photos, and some can be videos, which might explain why it seems off for have many photos have uploaded.
They are stored in both places until you delete them from your device. iCloud is used as a backup for your photos in case you were to accidentally lose your phone or have to restore the device. After your photos are uploaded to iCloud. Apple says that if you want to delete photos, you need to back them up to a computer before deleting. From what I understand the photos have to have a physical file one ONE of your devices in iCloud. You can set iCloud up in the Mac or PC to download the photos for you so you can delete them from your phone.