I've just read this question regarding why to sign only the hash of a message. I kind of understood the answer, but the question arisen is now:
Why can I encrypt any text messages with the same method while it is needed to split up the message in order to sign it?
I mean, signing is taking the private key and encrypting e.g. message A and on the other side taking the encrypted message and using the public key to decrypt it. What is the difference that makes the other way around (public key for encryption, private key for decryption) (more) secure?