Hi Kaan,
It is indeed not easy to reach the 10 GB size for an index, but yet not impossible. As you can read
here, "when an app tries to exceed this amount, an insufficient quota error is returned".
But before that, I would suggest to shard (divide or split) your index into multiple indexes. In order to do so, you should:
1. Create an additional index.
2. Store a new document in the second index if needed.
3. When you need to search for a document, search both indexes. For better performance, you can make asynchronous search calls.
According to what my colleague David (Google wise man) explained to me, this approach will not scale indefinitely. Beyond two or maybe three sharded indexes, this technique will become relatively inefficient. The Search API is not designed for infinitely large document collections, so if you plan to scale a great deal then you would need to consider alternative architectures. However this technique will at least allow you to double your search capacity.
Hope that info helps!