If "heaven" to you is the New Jerusalem, you are in error. Its dimensions--I have no reason to take them figuratively--are gigantic. While you are correct that few will go there, you need to revise your interpretation of the parable.
I do not know what "translation" you are using, but the text does not say that the kingdom of heaven is smaller than the mustard seed. The kingdom is like a mustard seed in that it begins small and grows large. Please read the rest of the verse (Luke 13:19).
A new translation comes out every so often, and a legitimate one would not make such an error. The "older book" you are reading reads like the author was careless in his use of the language. By omitting "like," he made a considerable error.
I brought up the verse that God's Kingdom was WITHIN you. Some person whom thinks they are a Bible Schalor told me that I didn't know what I was talking about and I didn't think I knew who Jesus was talking...