Quote Originally Posted by Tom Kalbfus View Post
That is how most of the creatures in the Monster Manual are presented. The creatures that have lots of hit points mostly have them because they are big, and not because they have advanced alot of levels. Most characters in most d&d settings are first level, the nonadventuring npcs are. So without having a high level, creatures have hit points based on their size and toughness.
Several problems, some of which relate to the post to which I am replying above, and some of which don't (but are still relevant):

First, you brought up strictly linking Hit Dice to size in the context of player characters, not monsters. In the editions where monster size sets Hit Dice (only 5e if memory serves), PC and monster Hit Dice and hit point progressions are built on very different mechanics, and in 3.5 monster Hit Dice are based on their type rather than size, so there's no particular reason why referring to how things work in the MM is relevant. At any rate, I would think you'd need more to go on than simply referring to the fact that 5e monsters' Hit Dice depends on their size.

Second, monster Hit Dice scale by size category in the 5e MM. "Big" in the context of monsters means things ranging from ogres to dire bears to dragons to the Tarrasque - it's simply a whole other scale than "this character is three feet taller than that one".

Third, in one post you mentioned not wanting to muck around with WP/VP mechanics on account of making the system incompatible with the Monster Manual (which one, by the way?). But later you're talking about handing out non-standard Hit Dice to creatures that are 10 feet tall and up, implying that you're now making your hit point system incompatible with the content of the Monster Manual. In other words, two of your apparent aims in this system appear to contradict each other.

Finally, in reality, being seven feet tall versus four or five feet tall doesn't really translate into being twice as difficult to kill: a blade between the ribs that gets a few inches in, or a slashing blow that catches the jugular, getting eviscerated, a stout clubbing that crushes part of the skull, or the physical trauma from a musket-shot penetrating one's vitals: all are likely to be instantly lethal, or very nearly so, to people in both size ranges.

I just don't see how the degree of granularity you're assigning to Hit Dice as a function of height is justifiable from the perspective of either mechanics or verisimilitude. (And it's clear from the bulk of replies that I'm not alone in this regard.)