There is something wrong with items on conveyor rails that have mass less than 20 kg. Consider these two:
The second item is 10 kg igneous rock, but it appears much bigger than another item on the left that is actually 20 kg.
Moreover it looses temperature much slower. Notice that it has temperature over 1100 C compared to the bigger item with only 200 C. So it's not just how it looks, there is something more. (the initial temperature of both items was about 1200 C)
Load the save file and look at both items on the conveyor rails.