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No one knows.

For regular reach weapons, your reach is doubled—so when you get bigger and your natural reach increases, your reach-weapon reach increases proportionally.

But whips are a weird exception: it just says it has a 15-foot reach. You could be overly literal and interpret that to mean that whips always have a 15-foot reach, no matter the size of the whip and wielder, but that’s remarkably inconsistent with every other weapon in the game, not to mention pure nonsense with respect to how whips work (then again, not being able to threaten with a whip is if anything even more nonsensical—“attacks of opportunity” are literally the only thing a bullwhip can actually be said to be good for in combat).

So no one thinks that the reach should stay at 15 feet. The question is, what does it become?

The first Google hit for this question is this Paizo thread, and literally the first two responses assert two completely different interpretations:

  • The whip gives you +5 ft. more than you would normally have with a reach weapon. So if a Large creature normally threatens up to 20 ft. with a reach weapon, with a whip it threatens 25 ft.

  • The whip gives you triple your natural reach. So if a Large creature normally threatens up 10 ft. without a reach weapon, it threatens 30 ft. with a whip (instead of 20 ft. with a regular reach weapon).

There is nothing in the rules that covers which of these two options is the case, and to my knowledge no Paizo developer has ever commented on the problem (James Jacobs has, but he is the creative director, not a rules manager—YMMV on his authority here). Ultimately, they may be ignoring the issue because whips are nearly unusable and so it probably shouldn’t ever come up.