I think the write action, will cause same size of data to write.
For questions like this, your best bet is to consult the AWS documentation. Check out https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/GSI.OnlineOps.html#GSI.OnlineOps.Creating for the suggested WCU settings for a GSI. If it’s lower than the original table, the index could lag behind in rebuilding.
Maybe it will replicate the data in a longer period, because it is eventually consistent.