Try to put your car apart in pieces, then ask a friend/professional to make it back.

Would you bet that everything will look and be exactly the same way it was before ...?

To some extent, what happens with the mail de- and re-construction is the same: expect a valid/correctly [re]built message, but don't expect it to be the very same as the original after rebuild. The 'glue' used to put things together is [probably] a bit different at reconstruction then the one used by the original message builder, which determines the slight difference in size.

If you're tempted to try, make a step further and apply the same procedure once more: de-construct your new message and then make once more a new one out of it.

Only now I would expect to not see any difference any more between the two new messages, built by the same tool from the same parts.