This video describes the basic information you need to start a NET memory investigation and in particular understand the data shown you in a .NET GC snapshot.
The first step in doing .NET Memory investigation is first determine whether .NET Memory is your problem and take a heap snapshot. This tutorial shows you how to determine how much of your process's memory is GC heap and (if that is your problem) shows you how to take a snapshot of the GC heap. For large GC heaps, PerfView does not dump every object, but instead samples the heap (but goes to great lengths to insure that sample is representative). This video describes this sampling and how to learn more about it.