Besides the cryptographic information about your actual key, the key files always contain some metadata, as to when the key was generated, whether it is packed, if so what kind of packing (base64 etc) was used to pack the key etc.
Try to open your key in a notepad to see if you can get any human-readable info.
Often you can read something like:
-----BEGIN RSA PRIVATE KEY-----
Proc-Type: 4,ENCRYPTED
DEK-Info: DES-EDE3-CBC,7BC....
JHQ8U0pDbeFM9h2jZSmiugxdqOa2q/MiX43Xa4Es6nKmzu9oI/ZfpIdAHi8qwtsD
mZ5bQRIXD9AXeIRy+0tG2ibUaphQEsvI995PWUsh8N9dVumsqykmMXSwND7tkbHB
iO/VVSAAD9bV3dbl5nbMwMnPG+YC3S90GAK4ZRIqrHRQ94fd/ZAvP8kV9ilwCmX6
swFlNBLGuKFllJ9qkyr+OOQqulrAyZAB2UThGCJJetELFtV4mLmIaHdgDIcUqpJp==
-----END RSA PRIVATE KEY-----
The two lines just after the beginning of RSA private key are metadata, that can vary in size.
If the key is in a binary format, you can usually get the meta information using some key management tool.