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Generally speaking, an .etl file is optimized for speed, not for size. If you are worried about size, compress the file. That said, if you flush the ETW buffers, a lot of zeros will be written to the .etl file. This may happen, for instance, if you make the flush timer (Session Properties) smaller than 1000ms.
When you enable compiler optimizations, especially with whole program optimization, the compiler enables automatic function inlining. When a function is inlined, the mapping between program code and debugger information is sometimes incorrect. This is similar to the effect when you step through optimized code. As a result, the function and line number information extracted from the PDB may be incorrect. You can compensate this effect by defining the macro WPP_EMIT_FUNC_NAME, however, the Jump To Source feature will not work correctly.