Normally a large tree is used to symbolize greatness. Think of a tree which you think would seem more fitting to compare the Kingdom of Heaven to than a mustard shrub.
I found it necessary to have an ash tree removed from my back yard this past summer. In spite of the fact that it was dying, because of its size and the fact that buildings had gone up around it since it was a sapling a century ago, it took a very large crane and a sizeable crew to get it out. Even that ash tree might seem a more ‘fitting’ comparison to the Kingdom of heaven than a mustard shrub. What do you think?