Size of encrypted files

[Preben Mikael Bohn]

Dear all, I'm trying to use gpg to encrypt files using only symmetric ciphers.
I'm using the following:

gpg --symmetric --batch --no-tty -z 0 --cipher-algo 3DES --passphrase-fd 0
plain.txt < passwd.txt

Can anyone explain the following:

1) How is passwd.txt being used in the encryption algorithms when used as the
above (as the key directly, padded with something, stripped off the 8'th bit,
etc)?
2) How come the encrypted file size is significantly larger than the plain
file? I would have expected it to be only slightly larger due to padding of
the plain-text.

Best regards Preben