How sure are you that the sponges are not going into some other water stream that's nearby, or that you're missing sponges while pick up? The items stack if you break alot around one another so you could have had the entities dropping at another area and then you expected them to be closer to you? I personally never had this problem and I break blocks fairly fast, would require more than one report to confirm this. Our despawn rates should be 5 minutes in a loaded chunk.