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