hogs do put out some heat. i grew up on a large scale hog farm, so i know a bit about them. my dad and uncle just sold it a year and a half ago, but my brothers and i managed it for the company that was leasing the buildings, for about a year or so after. in all of our grower and finishing houses, we did not have heaters and it was still high 70's to 80's in the coldest of winter (near zero F). if the power went out and the fans (pit and wall) were not running in the summer, the temp would go from 80's to 100's in a matter of 5 minutes. just recently, lightning struck and took out the warning/monitoring system and power to one of the barns that held 450 pigs. half an hour later (when it was discovered), there were less than 50 still alive. the automatic generator (capable of powering a city block) turned on, but the circuit panel was fried. pigs do put out quite a bit of heat.
size and age are not necessarily positive indicators that they are going to be tough and all fat. after they reach a certain age, you can stop feeding them "grower" rations and start with maintenance-sized rations. they will still grow, but not as quickly and not develop as much fat.
even if they do have a bunch of fat on them, it is easily trimmed off by your butcher. there will just be more waste. at a certain point you reach a stage where you're putting more money into them than you're going to get back out. but, on a small scale, it is not a catastrophe if they don't make market in a weeks time. you get 5, 10, 20 thousand hogs, and yes, you need to get them out before you loose your booty.
knowing hogs, though, i think it would be a mistake to leave them in the back of a pickup with any pickup topper that i've seen. they don't sit still and twiddle their thumbs when they get bored