Local disk storage in the temporary directory is an in-memory filesystem. Files that you write consume memory available to your function, and sometimes persist between invocations. Failing to explicitly delete these files may eventually lead to an out-of-memory error and a subsequent cold start.
You can see the memory used by an individual function by selecting it in the list of functions (https://console.cloud.google.com/getting-started) in the GCP console and choosing the Memory usage plot.
Do not attempt to write outside of the temporary directory, and be sure to use platform/OS-independent methods to construct file paths.
You can bypass the size restrictions on temporary files by using pipelining. For example, ...