You can actually see that this is going on with PGP, even with the padding.If you encrypt to 3 people, copy paste the result into a plaintext file.Now encrypt individually to 3 people. Copy paste and append the results together into a different plaintext file.Now compare the lengths. Even though it is the same information, it is clear that the first plaintext file is much shorter than the second one.Compression can only account for so much since ciphertexts are intended to look as much as possible like random noise to prevent cryptanalysis. So now you know why :-)