My understanding is that if a join will be made between two sets where at least one has VERY low number of rows, the CBO makes a Cartesian Join (Product) and then throws out the rows that don't meet the join condition.
In your case the CBO thinks the join is between sets of 1 and 82 rows each.
This can go horribly wrong if the stats on the tables aren't good.
(Of course you can get a Cartesian Product - and possibly blow your temp TS - if you forget the join condition!
If you didn't know already - a Cartesian Product of two sets is all combinations of rows from the two sets - can be very big.)