WIF Format and Storing Private Keys
The Bitcoin Core client stores private keys typically  in DER encoded format, at 279 bytes per key, for compatibility with older versions. There is no good reason for that though. For encrypted wallets, which are more modern, keys are stored using just the (encrypted) 32 byte secret plus the full public key. Compression is derived from the length of the corresponding public key, so it doesn't need to be stored separately.