well, you guys got me curious...
fwiw the 05+ extended cab tacoma is the same size, within inches, of the 00-06 extended cab tundra...wheelbase, width, length and height it's almost exactly the same truck.
that seems to be about where the similarity ends. the 00-06 tundra is rated almost 400# more load capacity than the 05+ tacomas, despite a very similar configuration.
perhaps toyota was feeling wary and underrated the tacoma, or they may have overrated the tundra. the tundra's entire drivetrain, save the v8 engine, came from v6 toyotas, including the transmission (despite the different part numbers)...the tranny is from a supra, and everything else is from the tacomas (literally, the parts are drop-in except for minor details on the v8 tcase due to the minor differences in the two v6 transmissions used in the supra and tacoma).
the -04 tacomas are very stout overall. toyota was apparently confident enough with the running gear that they wholesaled it to the tundra, with a higher gvwr.
dunno what all that might mean to you, or if it helps, or if it just makes things muddier...but there's the facts, for whatever theyre worth.
-sean