IIS Express runs x86 / 32-bit as standard, but there is also a 64-bit version.
The version used for all web projects can be configured in Visual Studio 2013 or 2015 in the Tools -> Options… dialog.
Under: Project and Solutions -> Web Projects
The setting is: Use the 64 bit version of IIS Express for web sites and projects
I encountered this while trying to debug some startup and initialization issues with IIS Express and Visual Studio.
The result from:
Was the path: C:\Program Files (x86)
But I was expecting the path: C:\Program Files
This was a clear sign that IIS Express was running as x86 / 32-bit.
Found the Visual Studio setting here:
Other possible solutions are to modify the registry or calling the desired version of IIS Express with a script.