If you have multiple classes with the same name from different classloaders and want to differentiate between those classes, you have to navigate to a specific instance, go to the references view of the heap walker and continue with the class selection as described there.
No specific view settings apply to the classes view.
Apart from the values for primitive arrays, all retained sizes are lower bounds. The algorithm is much faster than for a precise calculation and good enough in most cases. If you need the exact retained size on a per-class basis, make a new object set with the class of interest and click on the "Calculate retained and deep sizes" link in the object set description at the top.
java.lang.Class
objects of the selected classes.
After the inspection completes, the classes view will be shown again, with a class loader grouping at the top. You can select each class loader and see the loaded classes immediately in the classes table. By right-clicking a class loader row, you can select the class loader object and create a new object set with it.