Refining isn’t a pure process and then their is the mill scale and dirt from the millions on miles of carbon pipe and tanks that is it pumped through. If you could take all the dirt out of that filter and put it in a pile it wouldn’t be as much as you think for the amount fuel you ran.
Gasoline is somewhat cleaner because of the refining process but also doesn’t require as fine of filtering as Diesel engines do.
Bio diesel tends to be a bit dirtier also. Bacterial growth is also an issue that makes a huge black mess.