He doesn’t say future parts will be smaller than the first one at all.
Here is the full quote of what he actually says:
Nomura: (How about the next part?) Depends on how many parts the overall story will be. If we divide the story into large parts, it'll take longer to make. If we divide it into more detailed smaller sections, then developing it will be faster. I hope to release the next one ASAP.
So, he doesn't actually say that part 2 will be smaller than part 1 like the article suggests, but it can be inferred that it is being considered, since he says that smaller sections will be faster to develop, and that he wants to release part 2 ASAP.