Why are mango seeds so big?

My wife brought back a mango from the grocery store the other day.  Mangos are a fairly large fruit, but when I cut it open I find the seed itself is about 3 1/2" to 4" long based on the mango I had.

So I'm sitting here wondering why the seed is as big as it is?  My thinking runs along these lines:  Seeds of most fruits are smaller because the fruit itself "wants" to be eaten, go through the digestive track of some animal, and wind up in a pile of manure that will give a new plant - in this case a mango tree - a fertile place to grow.  But in this case the seed seems particularly hard to eat.

So I'm considering the unpleasantness of not only swallowing a mango seed, but also the unpleasantness of passing a mango seed - and I'm thinking something much larger than me must've at one time or the other led to such relatively huge seeds.   Admittedly, I guess it's possible that human cultivation has increased the size of the seed and as a parallel result the seed got bigger, but there are many large fruits which tend to have small seeds, so it seems unless the tendency for large seed was there to begin with that human cultivation would've gradually selected for smaller seeds when possible (or even seedless like grapes or watermelons). 

Now mangos are tropical, common in India, Bangledesh, and the Phillipines per wikipedia, so I'm wondering what huge tropical animal used to or currently eats mangos and can pass the seeds.  Elephants I guess come to mind, but why would a plant restrict itself to requiring animals the size of elephants to eat it.  It seems the natural course of events would favor smaller seeds in a fruit so more animals that are smaller than elephants could swallow and pass.  So I guess I'm a little confused by the large size of this seed.  It doesn't make sense that a super-sweet fruit that attracts insects and animals would have such a huge seed - and it's difficult to understand the benefit of the huge seed.