From what i have seen the IO's for the Indexes is small. The rule of thumb normally is big I/Os = small chunks; small I/Os = big chunks.
Seeing as th EV indexes normally produce small IO's then larger chunk sizes are best suited for it for performance
When accessting data files with lots of snamm IO's you want to maximize your IOPS, which ideally means sending each I/O to only one disk and spreading the I/Os evenly across the disks. What you don’t want is a single I/O getting sent to two disks, since waiting for the heads will slow things down. So you want a large chunk size. That large chunk will mean that most I/Os get serviced by a single disk and more I/Os are available on the remaining disks.