Some references on extents dealing with extent trimming, initial extent for parallel load, tablespace fragmentation, etc…
ASM AU Size And LMT AUTOALLOCATE
http://oracle-randolf.blogspot.com/2013/04/asm-au-size-and-lmt-autoallocate.html
Parallel Load: Uniform or AutoAllocate extents?
https://blogs.oracle.com/datawarehousing/entry/parallel_load_uniform_or_autoallocate
Tom Kyte covers this problem in great details in his post Loading and Extents
http://www.oracle.com/technetwork/issue-archive/2007/07-may/o37asktom-101781.html
Initial Extent Size of a Partition Changed to 8MB from 64KB After Upgrade to 11.2.0.2 or Later (Doc ID 1295484.1)
Bug 19912552 : PARTITIONED INDEXES CREATED WITH 8M INITIAL EXTENT WHEN _INDEX_PARTITION_LARGE
Segments of type INDEX PARTITION or INDEX SUBPARTITION will be created with 8M initial extent in version 12.1.0.2, instead of 64K in earlier versions.
Global indexes or indexes on non partitioned tables still have 64K.
