I would doubt that its 16" thick. Thats almost enought to
land a plane on. I would think the footers are 16" and
the slap itself is only 4" or 5"
Most if not all builders would not spend the extra money on
the extra concrete for no good reason.
If you are really pissed at the builders, get a
20 pound sledge hammer and see if you can
start to break up the edges. Its a good workout.