Can someone please explain this joke to me...?

LMAO!!! That is a good one. I have a feeling you already get it, but I will try to explain. Tonto is a native american in the Lone Ranger series. So the guy is insinuating that he is well endowed since she said Native Americans have bigger es. Also Goldstein is a popular Jewish last name. Saying he is also the best lover because he has Jewish in him.