Why are Photo/Video sizes on iPhone different than the sizes shown after imported to Photos app on my Mac?

Hi there. I've come across something I can't understand while trying to free up space on my iPhone X (13.5). I've imported all of my phone's photos and videos to my Mac for backup. While trying to figure out what pictures and videos I should delete from my phone, I've been looking at their file size on my Mac to see which ones are taking up the most space. I just deleted an especially large video that is over 1 GB, the way I've checked the file size is by (1) Selecting the video in the Mac Photos app and choosing "Get Info", and (2) finding that file in Finder, deep in the Photos library. However, when I deleted the video from my phone (and yes, I remembered to also "Delete All" from the Recently Deleted folder too) it doesn't free up an equivalent amount on my phone, it appears to have only freed up about 500 MB. Why would that be?

This discrepancy shows up in another place. If I go to Settings > iPhone Storage > Photos > Review Personal Videos, the videos are showing up far smaller on the iPhone than they are when I look them up on my Mac. I've attached screenshots from my phone and laptop for reference, the video shows up as only 684.2 MB on my phone, but is 1.51 GB on my Mac. Which one is right?? And why such a big difference between the way these two devices (both Apple) store these files? Thanks in advance for any insight!