1) I'm beginning to wonder if I am missing something in this question, or misunderstanding it; Maybe Jesus was telling them that the Kingdom was going to start small?
2) A mustard seed.
3) That it is so very tiny, compared to the huge plant it results in.
4) A rather large tree-like plant.
5) The prophecy in Daniel where a stone broke off of the statue and destroyed it, then grew into a mountain that filled the earth.
6) It's beginnings; at the start, it was small. But it rapidly grew into something thousands of times larger than it was.
7) Even though it may seem a very tiny, insignificant thing, it can grow into a mountain.

9) The lost.
10) I suppose how many birds it can hold.
It started small, and grew; eventually it will fill the earth. It's a place of safety too, no matter how small it may seem at times.