Also, note that a byte is different from a character. Because SMF stores "special" characters in a particularly space-inefficient way, each Russian character uses about 6 bytes, and each line break also uses 6 bytes.
Which would bring me back to something I've suggested earlier.
Another thing I've noticed is, that you're pretty "wasteful" in terms of lines with your BBCode, trying to compress would be another possible solution.
Although I don't think you would be abled to 'safe enough space' that way, in order to compress everything into a single post.