It's a quandary, isn't it?
It's why I personally recommend using a modified Savage Species reach system as a house rule.
Effectively, a reach weapon
shifts your natural reach by a certain amount based on its size.
A Medium reach weapon shifts your natural reach outwards by 5 feet. A human threatens one square at 5'; a human with a Medium longspear threatens one square at 10'. An ogre threatens two squares at 5' and 10'; an ogre with a Medium longspear threatens two squares at 10' and 15'.
A Large reach weapon shifts your natural reach outwards by 10 feet. An ogre with a Large longspear threatens two squares at 15' and 20'; a human Monkey-Gripping a Large longspear threatens one square at 15'.
Small reach weapons shift by 5'. Tiny reach weapons are special - they don't shift the reach of a Small or larger creature, but they allow a Tiny creature to threaten at 5' (but not in their own square).
It's not core, but it works for me... since the core rules don't really address reach weapons of inappropriate size.
-Hyp.