So is the high water mark growing faster than the actual data size? If you're processing lots of data the high water mark will rise - as will space you need to store the data!
If you're making extensive use of direct path inserts (which write above the high water mark) and regularly delete data you may end up with lots of wasted space. In which case you may want to reclaim some by shrinking/moving the table partitions https://asktom.oracle.com/pls/apex/f?p=100:11:::NO:RP:P11_QUESTION_ID:9527343800346989243 This deterioration was attributed to index fragmentation
How have you determined this?
Again, index fragmentation is only really an issue there is lots of free space that will never get reused. If this is an issue, like space in the table you can reclaim it with coalesce or a rebuild: https://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:9533003900346051914
Truncating a partition is a rather extreme solution (unless you genuinely want to remove all the data!)