How to union only those rows (from large table) with keys in left small table?

I have two dataframe, one is large, the other is small:

val small_df = sc.parallelize(List(("Alice", 15), ("Bob", 20)).toDF("name", "age")

val large_df = sc.parallelize(("Bob", 40), ("SomeOne", 50) , ... ).toDF("name", "age")

I want to add up these two dataframe but only those with the key in my small table, that is, I want my result to be like:

List(("Alice", 15), ("Bob", 60))

My first attempt is try to do union and reduceByKey, but I can't seem to find a way to union two tables and keep those rows with keys in the smaller one only.

Is there a way to do something like "left union" or other way to approach my answer?