It's a cluster so the settings are supposed to be the same across the board, that means you have to be configured according to the smallest indexer.
Increasing the index size will not cause issues with the data. However, It will have an effect on your disk storage consumption, your indexers might get full and it will cause your data to be frozen before the intended retention time to free up space. So it is important to understand the Splunk data lifecycle and set things properly like the data retention policy etc. (volume limits too)
The question is , does your smallest indexer can meet your 1 year retention policy ?
https://wiki.splunk.com/Deploy:BucketRotationAndRetention
https://docs.splunk.com/Documentation/Splunk/7.2.0/Indexer/Configureindexstorage
https://conf.splunk.com/files/2017/slides/splunk-data-life-cycle-determining-when-and-where-to-roll-data.pdf
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