Parallel query plans are slightly larger than serial query plans because they contain extra instructions on the partition or pages that the worker processes need to access.
During ad hoc queries, each worker process needs a copy of the query plan. Space from the procedure cache is used to hold these plans in memory, and is available to the procedure cache again when the ad hoc query completes.
Stored procedures in cache are invalidated when you change the max parallel degree and max scan parallel degree configuration parameters. The next time a query is run, the query is read from disk and recompiled.