Postfix mail size counting

If I'm right Postfix counts mail size with an attachment sent to multiple receivers as one big e-mail.

Eg.: User sends mail with 8 MB attachment to two receivers. Postfix thinks it's 2 x 8 MB = 16 MB mail. And when message_size_limit is by default 10240000 bytes ~ 10 MB it rejects it.

My question is: Is there a way to tell Postfix to count the attachment only once? I'd like to avoid setting bigger message_size_limit.

Your assumption is not correct.

Postfix stores messages to multiple recipients in a single queue file, which is why you can see 250 2.0.0 Ok: queued as 99BF8300096. It does not increase the size for the file as a multiple to the number of recipients, though multiple recipients may cause (slightly) more envelope (header) information.

message_size_limit (default: 10240000 bytes)
    The maximal size of a Postfix queue file, including envelope information (sender, recipient, etc.).

Edit. This is pobably what you're mistaking:

[t]he actual length of MIME-compliant Base64-encoded binary data is usually about 137% of the original data length, though for very short messages the overhead can be a lot higher because of the overhead of the headers. Very roughly, the final size of Base64-encoded binary data is equal to 1.37 times the original data size + 814 bytes (for headers).
-- http://en.wikipedia.org/wiki/Base64#MIME