Probably because its brakes are designed to stop something like 15 thousand pounds.
D1 brakes aren't bad, but nothing to brag about. I find it funny when people complain about RRC brakes and want to swap the entire setup to D1 vacuum servo - RRC brakes are vastly superior, and [when the pump works] they can be used with a dead engine.
That all said, when you upgrade the calipers to 110 front and rear (even with the same rotors), and fit a proper master cylinder (even to the same vacuum servo), you should see a great improvement.
To give you an idea - my prop valve seized about two years ago, and I drove the D1 on 31.5" tires for a year without even knowing I had no rear brakes at all.
I am pretty familiar with the how's and whys.
the f-350 certainly has bigger brakes, though they are actually very much undersized for the rated combined weight. Ford started getting serious about brakes *after* the venerable 7.3 powerstrokes were retired.
The volume and area ratios for those particular brakes are actually less favorable than the D1's.. the key difference is the leverage boost provided by hydraulic boost vs. Vacuum boost. Also, it is worth noting the hydraulic booster has more *dead engine* reserve boost that the stick vacuum tank. Indeed, with a stickshift (as my d1 is) the power steering pump can continue to generate pressure as the wagon decdecelerate.
The D1 brakes are garbage. You can get by with them if you don't tow and if you don't try to haul it down from 80mph. As per another comment in this thread, the disks and calipers are D90s rather than 110s I believe. I included a link to them as well so please correct me if you know otherwise
the keys updates are: I put ventilated front calipers & rotors on both axles and I am installing hydraulic boost.