Index Partition Size Double of its Table Partition?

We're trying out a new non-unique covering (including) index on a couple of
table partitions. We put the index on partitions for last month and this
month. Both table partitions have similar sizes (45-46 GB) and row counts
(330-333 million). The covering index on last month's partition is 50GB,
but this month's index is 79GB already. The table is basically write-only
as well. So there shouldn't be any real bloat here.

One thing worth mentioning is that the table is 4 columns, the index is on
two of them and includes the other two. I can't think of an explanation for
the index being so much larger than its table, especially compared to last
month's index.

Curious if anyone has any thoughts on what might be causing this.
--
Don Seiler
www.seiler.us