From what I understood, compressed private keys are supposed to represent the same secret key, stored more efficiently thus taking up less space. I assume 'more efficiently' has to do with secp256k1 specifics that I'm not aware of, but I'll take that for granted.
However, when I compare a few compressed and uncompressed private keys:
# example private key hex: 85B7DFAB8D2C695C0EBF013AD50A758FDBC62775391B7CE33282634F7DB47480 uncompressed: 5JqBEaUuYJFA4dQsd62ys13RumxCGu97BRu3LzwaVduD8MQnFwc compressed: L1heAvnAu97V11iNSHDjyhBiwwQcmB8nH2thMGjtqPsSXKaZ9VwY # another example private key hex: 6190289CEB09ED41776AE27AE81F46C6540C56CF8F3900BCAADE96B4470F23D6 uncompressed: 5JZFhabXLayCMCehpyhdCPZ5LXwvgvu3gX4Png4xYzCWYCXJ3jx compressed: KzVMt7HNuSw36ARRCmGJmGYa9DbRyEtiqRfyabSUCBToNRwMrAog # yet another example private key hex: 763B1998BF7282A99CA21EB821EB9A204589FFA189E290EE64528854FDA91080 uncompressed: 5JiMdW1Czd3tTuy33wiC5wmrkhZxx2PNcC7NVZoFc7AixTVgmf5 compressed: L1BY471uwL7k9nnL3zUovpkzbt4Vws7kNL37s5m9TxDvBjnadvMy # etc
I notice that the compressed version (at least the WIF/base58 representation) of a key always seems to be longer. Why's that?