This is one of my favourite all-time novels, so well done for studying it! You have asked a very important question, as obviously the title that an author gives to their work is a very important decision to make, and clearly it must link in somehow with the overall theme or message of the book.
One place to start would be looking at Chapter Eleven, which itself bears the same title as the title of this great book. One of the things that is described in this section is the coming together of Ammu and Velutha:
Who was he, the one-armed man? Who could he have been? The God of Loss? The God of Small Things? The God of Goose Bumps and Sudden Smiles? Of Sourmetal Smells - like steel bus-rails and the smell of the bus conductor's hands from holding them?
Considering this quote, as the series of rhetorical questions thinks about the identity of Velutha, we can apply the title of "The God of...
(The entire section contains 479 words.)