Hello Akash,
I understand that you want to know if there is any optimization happening in the background on your indexes in Search API or indexes in general. Is this issue causing concern for your project at the moment?
I have researched the behavior you reported. I have not found documentation suggesting that the index size could change on its own or is changed by Google.
In this case, if you have reason to believe that this size change was not initiated by one of your actions, I invite you to create an issue with Issue Tracker so that we would be able to look into it.
Note that Google Groups are reserved for general Google Cloud Platform-end product discussions and not for reporting issues, which is why I suggested moving the troubleshooting to Issue Tracker.