Great article on it here:
http://truckyeah.jalopnik.com/truck-sizes-classification-explained-from-tacomas-to-1613958192
Tacoma is Class 1 any way you look at it, because of GVWR.
Highlights below:
The US DOT puts trucks into classes by "Gross Vehicle Weight Rating" (GVWR) ranked from 1 to 8 (smallest to largest).
GVWR refers to the maximum operating weight a truck can possibly carry while driving including the truck itself. GVWR classes have nothing to do with what parts the truck is fitted with, how beefy the suspension is, or what the truck looks like. They are solely based on weight.
Weight category definitions from 49CFR565.6 (2000)
As you saw above, pickup trucks live in in Class 1, 2, and 3. Class 1 trucks are very small, pretty much only the Toyota Tacoma and Nissan Frontier are there at this point. The Ford F-150, Chevrolet Silverado 1500, GMC Sierra 1500, Ram 1500, Toyota Tundra, and Nissan Titan fall into Class 2.
Manufacturers also consider a "Class 2B" (GVWR 8,500 to 10,000 pounds) because 8,500 pounds is the cutoff at which they no longer have to provide EPA MPG estimates on window stickers. This includes "heavy-duty pickups" like the Ford Super Duty, Chevrolet and GMC 2500s, and Ram 2500s. "3500" pickups are basically all Class 3 trucks as far as GVWR is concerned.
Trucks were initially advertised based on payload because they "competed directly against horse-drawn wagons" which is pretty wild to think about.
The classes that stuck were "Half-Ton," "Three-Quarter Ton," and "One Ton." Those designations came about years ago when each manufacturer pretty much offered three trucks and those were the minimum payloads they were respectively capable of.
These classes are confusing now because they're pegged to capability-levels modern vehicles have long surpassed. For example, a 2014 Ford F-150 is considered a "half ton" truck but most variants can carry a lot more than 1,000 pounds in the bed.