Very large subpartitions after table redefinition
Yeah, it is recurring problem.Starting with 11.2.0.2, the default segment size of partitions was increased from 64K to 8M. This means, even if your partition is empty, it still consumes 8 MB space right after its creation.

Initial Extent Size of a Partition Changed to 8MB from 64KB After Upgrade to 11.2.0.2 or Later (Doc ID 1295484.1)

You should specify the parameters you mentinoned when you create the new table, for example: INITIAL EXTENT 65536 NEXT EXTENT 65536

Alternatively you could set a hidden parameter, so the database reverts to the old behaviour, session or database level:

alter session set "_partition_large_extents" = false;
alter system set "_partition_large_extents" = false;


But I would suggest specifying the extent sizes manually over modifying a hidden parameter.