I found your logs. (We have them here too, which is at least a tiny fraction of that 53 GB)
For now I’d recommend stopping Tailscale when not in use via iOS (in Settings > VPN and flipping the “Status: Connected” switch), not via the Tailscale app itself until we figure out what’s going on. (The iOS Settings way stops it much more aggressively.)
One thing I see is that one of your machines in particular (NUC) but really all of them to a lesser extent seem to be flipping back and forth between London and Frankfurt as their nearest “home” connection, even though I thought our code did a good job at preventing such oscillation. That then means they’re sending your phone a bunch of updates about where their new location is. At least in the next Tailscale release, that bandwidth is smaller. But they shouldn’t be flipping back and forth between London and Frankfurt anyway.
While that’s inefficient, it doesn’t explain 3.5 GB in a day. In the past 72 hours there have only been 300 flips back and forth.
I’ll look into it more with colleagues either tomorrow or next week.