I created a wallet called ”Test”.
Two files were created (Test and Test.keys).
According to https://monero.stackexchange.com/a/3123/11776, Test is the wallet cache.
What is the purpose of this cache and does it change significantly over time (does its size remain relatively constant over time)?
The cache contains data that's retrieved from the blockchain. It includes the hashes of blocks (used to detect reorgs), then data for every transaction that you sent or received, custom information like user tx notes, secret transaction keys, that kind of thing. It will grow as you transact in Monero.