Add effective size vs. size explanation to docs

docs/memory-infra/README.md

 # MemoryInfra
 
 MemoryInfra is a timeline-based profiling system integrated in chrome://tracing.
 It aims at creating Chrome-scale memory measurement tooling so that on any
 Chrome in the world --- desktop, mobile, Chrome OS or any other --- with the
 click of a button you can understand where memory is being used in your system.
 
 [TOC]
 
 ## Getting Started
@@ skipping 75 matching lines @@
      non-Blink objects.
  * **PartitionAlloc**: Memory allocated via [PartitionAlloc][partalloc].
    Blink objects that are not managed by Oilpan are allocated with
    PartitionAlloc.
  * **Skia**: Memory used by all resources used by the Skia rendering system.
  * **SQLite**: Memory used for all sqlite databases.
  * **Sync**: Memory used by Chrome Sync when signed in.
  * **UI**: Android only. Memory used by Android java bitmaps for the UI.
  * **V8**: Memory used by V8 Javascript engine.
  * **Web Cache**: Memory used by resources downloaded from the Web, like images
-   and scipts.
+   and scripts.
 
 The **tracing column in gray** reports memory that is used to collect all of the
 above information. This memory would not be used if tracing were not enabled,
 and it is discounted from malloc and the blue columns.
 
 <!-- TODO(primiano): Improve this. https://crbug.com/??? -->
 
 [oilpan]:     /third_party/WebKit/Source/platform/heap/BlinkGCDesign.md
 [discardable]:base/memory/discardable_memory.h
 [cc-memory]:  probe-cc.md
 [gpu-memory]: probe-gpu.md
 [partalloc]:  /base/allocator/partition_allocator/PartitionAlloc.md
 
+## 'effective\_size' vs. 'size'
+
+This is a little like the difference between 'self time' and 'cumulative time'
+in a profiling tool. Size is the total amount of memory allocated/requested
+by a subsystem whereas effective size is the total amount of memory
+used/consumed by a subsystem. If Skia allocates 10mb via partition_alloc
+that memory would show up in the size of both Skia and partition_alloc
+but only in the effective size of Skia since although partition_alloc
+allocates the 10mb it does so on behalf of Skia which is responsible
+for the memory. Summing all effective sizes gives the total amount of
+memory used whereas summing size would give a number larger than the total
+amount of memory used.
+
 ## Related Pages
 
  * [Adding MemoryInfra Tracing to a Component](adding_memory_infra_tracing.md)
  * [GPU Memory Tracing](probe-gpu.md)
  * [Heap Profiler Internals](heap_profiler_internals.md)
  * [Heap Profiling with MemoryInfra](heap_profiler.md)
  * [Startup Tracing with MemoryInfra](memory_infra_startup_tracing.md)
 
 ## Rationale
 
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 Chrome-side design docs:
 
 <iframe width="100%" height="300px" src="https://docs.google.com/a/google.com/embeddedfolderview?id=0B3KuDeqD-lVJfndSa2dleUQtMnZDeWpPZk1JV0QtbVM5STkwWms4YThzQ0pGTmU1QU9kNVk">
 </iframe>
 
 Catapult-side design docs:
 
 <iframe width="100%" height="300px" src="https://docs.google.com/a/google.com/embeddedfolderview?id=0B3KuDeqD-lVJfm10bXd5YmRNWUpKOElOWS0xdU1tMmV1S3F4aHo0ZDJLTmtGRy1qVnQtVWM">
 </iframe>