mounted combat and reach

[Titania, the Summer Queen]

If you are wielding a reach weapon and riding a large creature, do you threaten out 10 feet from the base of the mount?

What if the mount is huge?

[Michael Sayre]

Yes. Your reach always stretches out to 10 feet, regardless of your mount's size. You are just assumed to occupy whatever squares your mount occupies, this does not change your actual size.
When mounted, you are essentially Schroedinger's knight, assumed to occupy every square your mount does, and no specific one of them.