There are several issues that can impact the rated capacity. One is the ability of the engine/tranny combination to generate enough useable power. Most of the time in light trucks (pickups), the higher GVWR/GCWR comes with an automatic transmission because so there is far less risk of an inept driver doing damage. The carrying capacity of the suspension and tires is a big variable, of course. Another issue is the ability of the steering, and more important the brakes, to handle the load.
I regularly see people who got crummy advise somewhere pulling trailers or carrying truck campers on pickups that are far too light. If you are going to just use it as a daily driver, do what appeals to you. If you are at all serious, get the biggest, most carefully and heavily spec'd truck you can. There are many reasons for the growing trend toward buying used Class 8 trucks to pull RVs - engine, suspension, and brakes are among them. If you know what you are doing, you can get a good used semi-tractor for less than a pickup appropriate to that use, and be a LOT safer. And for pete's sake, if you pull a trailer, use a 5th wheel. Ball hitches, even with load equalizing systems, suck to a serious extent. The pivot point is all wrong for the combination, leading to serious safety hazards, and without the trailer, the ball probably blocks your rear license plate, which is a sure way to get a ticket.