Here in the UK, I have an above ground patio pool (30" X 12") with three goldfish. Of late one of the fish is constantly with one of the other fish, almost attached. Whilst I anticipate the reply will be "they may be getting ready to mate", the chaser is half the size of the one being chased. If they have young, are they likely to survive and how many should there be.

A few weeks ago, we treated the water because the fish appeared to have white spot, but that's clear now, and of late i've used greenaway with not much success. ther is no filter, just plants.

If you have enough plants for the young to hide in from the parents (who might eat them), they may well survive. One possibility other than mating is that the larger fish has some parasites that the smaller one is trying to eat.