Hi Rytis,
Just wait for few more years, fruit size will increase. Fruits in the mother tree weigh around 3/4kg. Thickness of flesh in arils vary from tree to tree. Taste is also not predictable in seedlings. My seedling have lots of fruit in the lower branches that I can pluck by hand from ground. Fruit size is just ok with very few arils inside! I hope fruits will be better this year.
One more thing I want to point out is that the fruit shown the picture is not fully formed. This usually happens in younger plants and in the absence of adequate pollination.
So, my point is that don't rule out your Wild jackfruit plant, if the taste is good. Taste is not going to change that much in years; but rain during fruiting season can definitely ruin the party.