As Saleem indicated, the pxdecisionresults_* files represent the "decision log". Removing them is not recommended, as this will stop your adaptive models from learning from new responses. There's a tutorial on how this works: https://academy.pega.com/library/72/implementing-adaptive-decisioning.

The life span of records in this dataset is governed by a "Time to Live" setting in the strategy shape in the data flow. This is a Cassandra property (TTL) and Cassandra will periodically purge data that is marked to be no longer valid. I believe the general recommendation is to have twice the amount of disk space available as might be needed for the data itself. 

I don't know which version of Pega you're on but in some version there was a bug where this TTL was not honored and data would grow. There is a hot fix for this - Pega Support should know more: SE-40749.