User @alecks answered this for me: the 'trash' folder in Google Photos was the culprit of taking up space.
I had one video file in there from my phone accidentally captured 3 minutes of video in my pocket, taking up hundreds of MB. I deleted it (and a bunch of other photos) and now the app is taking up 400+ MB instead of 0.97 GB.
TL;DR - if your Google ...
There's also Motion-Photo-Viewer. Can either be self hosted or used online to play a single *.MP.jpg (and extract the mp4 through right-click-save-as.
You do not need a Web Server, so cloning the project and opening index.html will suffice.
I tried out the answer from Kory above - thanks Kory. Since my Pixel 4a 5G adds a significant amount of debug info between the jpg and the mp4, my extracted mp4's didn't play.
So I had to add a little bit of extra functionality to Kory's solution
/** execute like > php motionPhotoSplitter.php "Camera Roll/*.MP.jpg" < */