the 40" xzl are light i think they weigh in around 250lbs maybe a little more
..........compared to my 52" xzl 400lb tire/45lb rim per corner and i cut lugs off to get the weight down
i think a 20" rim with a good tire combo can come out alright, but it has it's limit. you plan to get serious and or apply the truck to offroad conditions than a 15-17 rim is o.k. so that way you can get a tire with a side wall.
i have an 04' titan and some the 20" rim/ tire combos look good for them i am fixing to do a 20" rim with a 305/55 nitto grappler a/t, i'm sure this won't hold a candle light to old boggers i had on my 71, but i don't get to crazy with my titan (family truck) thats way i have my project truck
or you can go this way like my buddy, his truck has 20" rim with a 38" tire
he was running the 46"mich but swapped out for the weight loss to horse power ratio.....