By RAW, Tiny creatures don't get any benefits from reach weapons. A reach weapon (usually) doubles the wielder's natural reach, and 0 ft doubled is still 0 ft. The SRD doesn't come out directly and say that Tiny creatures don't gain benefits from reach, but I think it actually is RAI. For example, note how in the Big and Little Creatures In Combat section, they don't talk about reach weapons when discussing Tiny creatures, but they do when discussing Large creatures. That makes me think that RAI is for Tiny creatures to not gain additional reach from a reach weapon.
The Rules Compendium actually does say it directly, on page 150: "Tiny or smaller creatures gain no advantage from reach weapons." So, if your DM is using RC, you're screwed.
The whip, of course, is the oddball: someone else will have to tell you how whip reach interacts with wielder size, because I don't know of any official rulings on that.