What is the lightest browser to launch a jupyter notebook?
Try 32-bit Chrome or Firefox, it may significantly lower memory usage.

Remember, it is surprisingly difficult to estimate physical memory consumption of given app: https://dzone.com/articles/windows-process-memory-usage-demystified

Also - unused RAM is wasted RAM. Do not blame browser for taking and using, what's available. When overall memory usage will raise, browser can drop some internal cache and free that memory.