The game downloads a ton of images, texts, audio clips, models, icons and textures once you open it for the first time. On my device, the estimate is 350 MB for that (don’t have beta access).
I’m not sure why is it loading additional files continuously, but we have seen reports from other players for the increased data usage.
Right now, I don’t know what to answer you honestly – yes, Niantic will probably optimize Wizards Unite data usage and cache assets in a smarter way, but we have no idea when or how this will be implemented.
We know this happened with Pokemon GO throughout the game’s life cycle, the game has improved dramatically nowadays.