Are you joining on a field that is less than unique - for example if you join on a department that appears in many rows you will produce a number of rows that is a product of the combinations. This would happen in a SQL server join as well.
If you can't join on something unique, then maybe data blending in Tableau may be a better option, or maybe custom sql and a union - really difficult to say without seeing your data sets.
Yep, that's it. The values in my field aren't unique, so that would explain it. Thanks a lot!