I understand that fruits like the avocado and the osange orange are so large and unwieldy that their evolutionary "intent" was to be consumed whole by megafauna that are now extinct. What I don't understand is why there was selection pressure for such a relationship to develop. Wouldn't it be to the plant's advantage if its seeds could be dispersed by more types of animals? What advantage does the plant get by limiting seed dispersal to a small set of large animals?
There are a few advantages to megafauna:
- they travel large distances and are hence are likely to be able to spread your seeds widely.
- They also eat large amounts, so can carry a lot of seeds and distribute those around widely.
- Large fruit capable of being eaten only by the largest fauna might have the seeds survive by not being eaten and destroyed by smaller animals
- Large animals have large guts, so the more of the seeds might survive undamaged
Another significant factor is that a megafauna animal also produces mega-feces, literally a large pile of nice warm fertilizer for the new plants to grow in.
You may be focusing too much on the megafauna. If there are selective advantages to making a big seed, some seeds at least will get bigger and bigger until there is no animal that will disperse them. If you've ever sprouted an avocado seed, its shoot and first leaves grow very high very quickly. Smaller seeds may have more dispersal routes, but if they end up near an avocado seed, they will have to grow in its shade.
Seeds may continue to get bigger past the point that they can be swallowed whole by an animal, if simply dropping the seeds is viable (Quercus insignis acorns can be up to 8 cm in diameter), but then there will be selective pressure to reduce the energy spent on the fruit coating the seed.