Hi Seaman, yes you have to fill the firebox, the deeper the better, Coal likes company,, a 'community fire' each piece radiates heat to it's neighbor, and visa versa, the whole coal bed works together to burn at it's best.. If you take a shovel full of happily burning coal out of the fire, it will soon die.. It needs the coal bed to burn well..
Many people will not only fill the firebox to the top of the firebrick, but mound the coal in the center to get as deep a fire as possible. Unlike wood, this will NOT cause a runnaway fire from too much fuel.. coal's burn rate and heat output is easily controled with the combustion air draft controls..