--memory-limit
The Rerun Viewer can not yet view more data than fits in RAM. The more data you log, the more RAM the Rerun Viewer will use. When it reaches a certain limit, the oldest data will be dropped. The default limit is to use up to 75% of the total system RAM.
You can set the limit with the --memory-limit command-line argument, or the memory_limit argument of rr.spawn.
Alternatively, you can adjust the limit in the Viewer's settings.
This setting is remembered between sessions, but --memory-limit overrides it, and rr.spawn always passes that flag (defaulting to 75%).
To find out what is using up the memory, see Diagnose latency and performance.