Too thick, too thin, grabs the wrench, wrench falls out???
Most combination wrenches are "average" thickness. The Craftsman/SnapOn standard if that's what we should call it.
But there are thin, which are very handy. They will fit the Ernst type I referenced but won't be retained as well or may not release as well.
There are also very thick types, not common, which may or may not fit.
For most users, they are fine and really do their job - keeping your toolbox neat and organized.