After I encrypt and decrypt same file, it is a bit larger than original. Why?
It is because all ciphers work on block of data. When you encrypt your file, last block usually has few bytes padded (up to BlockSize value) so that cipher works on complete block. Naturaly, when you decrypt that file, you get few bytes extra garbage at the end.
To get rid of it, try using Optimized property set to True, or use PKCS7 padding.