Lots of fruits are intended to attract animals to eat them, not just avocados or large fruits.

Seeds do not need to be large to survive the gut, for example if you eat corn kernels they will easily survive your gut.

And the spreader animal does not need to eat the fruit whole to ingest seeds, it can just take bites out of the fruit and then ingest some seeds whole incidentally.

A pig or a cow, for example, will eat a pumpkin. Not whole, of course. But in the process they will end up ingesting whole seeds, that will survive their digestive tract and be spread around.