Pandas Left Outer Join results in table larger than left table

Question

From what I understand about a left outer join, the resulting table should never have more rows than the left table...Please let me know if this is wrong...

My left

Answer 1

A small addition on the given answers is that there is a parameter named validate which can be used to throw an error if there are duplicated IDs matched in the right table:

combined = pd.merge(a,b,how='left',left_on='id',right_on='key', validate = 'm:1')