You do realize "slowed" means your speed gets cut down to 2 squares? IE you're carrying something so heavy that you're moving less than half your normal speed?
Yes.
1) "That number" came from a 4e warhorse's load figure (18 x 10) which was multiplied by 4 to produce 720. I suggested this because it was done in 3rd edition, and in 3rd edition horses didn't get slowed to less than half speed because a big dude jumped on top.
Actually, in 3.5 the proper multiplier for a Large creature is 3x, not 4x.
And the strength of a warhorse in 4E is 21, not 18 (and a riding horse is 19).
And you can't compare the
normal load of 4E to the
light load of 3.5. 4E has simplified loads down to normal, heavy and drag and gotten rid of the medium and stagger weights. Based on the numbers, the normal load is heavier than the light load and lighter than the medium load.
Thus, the 4E normal load of a horse is 210 pounds while the 3.5 light load is 153 pounds and the medium load is 306 pounds. So to apply the 3.5 formula to the 4E numbers, you need to take roughly 3/4 of the normal load and multiply that by 3 - or, essentially, double (or, to be more precise, take 2.25x) the normal load to get a 3.5 quadrupeds normal load (and double that again for their heavy load).
This gives you around 450 pounds for the 3.5 'heavy load' of a 4E heavy warhorse.
And, of course, it is a fallacy to use the 3.5 multiplier with the 4E Str because they most likely increased the strengths of the quadropeds to get them to the final carry weight they wanted using the 4E formula (aside: quadruped carry weights are increased 25% in 4E). If you use the 3.5 str (which is 18) and do the same process you get a carry weight of 405#.
Thus I can see a lot of numbers being possible, but none of them are 720.
But of course, that is all based on the questionable assumption that horses can carry three times as much as a human of the same strength and it requires ignoring the fact that they most likely choose the strength value of a horse to end up at exactly the carry value they wanted in the first place (in both systems).
But the bottom line really is: Is the weight limit they put in place logical considering real world limits? (Yes) Does it make sense as a game restriction (Yes). Does it fit with the genre and its expectations (Yes). Does it impose restrictions on the character that make the game more fun(Yes). Was it silly for a 3.5 horse to be only slightly slowed carrying 900# (Absolutely)
2) Says who? I'm a weightlifter. I weigh 185 lbs. I've calculated my 3rd edition Str score at 14, but 4e is much harder to gauge with "real life" so I honestly don't know; we'll just say it's 14 for argument's sake. So it stands to reason that a guy with 16 strength (your average human fighter) is going to weigh at least 200 lbs, if not more.
Also, dragonborn are not large.
Your weight logic isn't. But I don't argue that a human can easily be over 200#. Heck, I'm over 200#.
And the
lightest adult dragonborn weighs in at 220 (as per the DMG) and their weights go up to 320. They may not be 'large' as a specific game term. But they average 100# heavier than a human.
Just out of curiosity, how fast do you move carrying 140 pounds?
3) This point invalidated the rest of your post entirely. If you're going to just slap on an extra hundred pounds of carrying capacity "just because" then why are you even arguing about it? That's silly.
I'm not 'slapping on an extra hundred pounds of carrying capacity. I'm making a ruling on a fact that is not in the books, based on given information in order to create a ruling that is consistant with the rules as they exist (including the fact that Rage Drakes appear in the Dragonborn encounter and are mounts according to the MM and the fact that horses are not appropriate mounts for dragonborn - as already noted in this thread).
If I need a justification it would be that their different body shape (more squat, lower to the ground and - very importantly - massive legs compared to the legs of a horse) enable them to carry far more weight. (Horses have very thin legs for their size which is why they are so susceptible to leg injuries such as befell Eight Belles.)
"Slapping on extra carrying capacity" is what you proposed.
I hope you can see the difference.
Carl