I just noticed this that items that are selected to be composted aren't being broken into 20 kg packets when shipped manually via a conveyor loader. I had a dupe load up a bunch of extra balm lilies to be composted (98 kg) and i noticed my loader only sent one packet. When I noticed it didn't split it up I paused the game and noticed that it sent a 98 Kg packet.
Use a conveyor loader to ship compostables. Notice that it doesn't split the compostables into 20 Kg packets.