Stone compaction on placement
You are correct, compaction is very important in order to control future settlement. 

Generally, we require a minimum of 95% compaction under a “Proctor” test that governs moisture content, maximum of 6” depth lifts, etc. 

Remember, compaction is not just weight...vibration is important too. 

So, to answer your question, if gravel is dumped at approximately 80% compaction, then there’s a difference of about 15% settlement. (This is highly dependent on the type of gravel, manner in which the gravel is deposited, etc. )

Here is an article that explains it:
https://www.multiquip.com/multiquip/pdfs/Soil_Compaction_Handbook_low_res_0212_DataId_59525_Version_1.pdf