In addition to somesoni2's comments, how much % disk used do you have on the existing indexers?
While initially new data / replicated data will be added to the new indexers, there are a few scenarios that can trigger a data re-balance which will even the spread of data among the cluster members.
So when you do the calculation, let's say you have 10 nodes of 1024GB each currently (total 10240GB), and you adding 2 800GB nodes.
If your using 9216GB already in the cluster (90%) and you add 2 more then the total is 11840GB available.
So you could end up with 768GB/indexer which would make your new indexers close to the limit before they accept new data...
Obviously the above maths is a very simple example and the data balance is never perfect so you would want some space left for contingency.
Could you provide little more details about your environment, specifically on Indexer side? Do you use Indexer cluster? Single-site/Multisite? If yes, what are the replication factor and search factor for it?
It is strongly recommended that you provision all peer nodes to use the same amount of disk storage. Saying that it may still work fine with no major issues, as long as new nodes always have sufficient storage to handle incoming data and replicated data. I would setup an alert to monitor disk space on peer node to ensure get alerted before time, in case those low storage peers are near full.