Little Snitch Network Extension consuming gigabytes of memory is not normal. This issue is a memory leak in Apple’s framework, affecting macOS 15.0 Sequoia. Running the leaks command in Terminal can determine if you are affected. While macOS 14 Sonoma may have a few kilobytes leaked, macOS 15 Sequoia can reach hundreds of megabytes. Apple has been notified of this bug (FB15552991), and users are encouraged to report if affected. To temporarily fix the issue, restart the Network Extension through Activity Monitor. Hopefully, Apple will prioritize fixing this issue in a future macOS update.
https://obdev.at/blog/a-memory-leak-in-apples-network-extension-framework/