I didn't make this change lightly, and it happened gradually, as I struggled with the Herculean task of defining ability score adjustments for animals ranging from mouse-sized to whale-sized. What the heck would the Str adjustment for, say, an elephant be, vs. the Dex or Con adjustment compared to the human baseline assumed by the B/X rules? How about those aforementioned mice, or whales, or horses? What about a damned T. rex or a griffin? The vast variety of animals (mythical and real) meant it was going to be a long, hard slog, and the prospect of tackling it nearly made me abandon this project altogether.
The root of the problem was that pesky human baseline assumption. The OSR game rules assume that all characters are going to be obligate bipedal humanoids, with all the anatomy and musculature that entails. And that's a problem when writing a game about the rest of the animal kingdom because, frankly, us obligate bipedal humanoids are a bunch of mutant freaks compared to even our nearest kinfolk, the apes and monkeys. The capabilities granted to us by that mutant anatomy are, in many ways, a great restriction on character versatility (how many longpaws can fly at 1st level like a bat, an owl, or a raven can?). A game designed around us couldn't handle the majority of the world's population very well. Or so it seemed.
Let's face it: most animals aren't tool-using bipedal humanoids, but they can do a lot of amazing things naturally that longpaws would have to use magic to achieve. If I wanted to make this game work, I needed to shift the focus off of the freakish mutants who've been hogging the game table all these years and ask myself: what would the B/X game look like if it didn't assume all characters were freakish mutants who walk on their hind legs and use their front paws mostly to play with sticks and rocks?
I started, at first, just focusing on defining and/or expanding what particular types of animals could do within the rules, based on their "monster" entry in some old-school or OSR source, and leaving the great dragon of ability score adjustments for later slaying. This was, originally, just a tactic to get my butt writing something for the game.
But it turned out that focusing on bestiary stat blocks was the key to the whole project, because monsters (including animals) run just fine without ability scores of any sort. That point didn't really gel in my mind, though, until I came upon the "monster" entries for freakish obligate bipedal mutants like the Bandit, Berserker, Dwarf, Elf, Halfling, or the generic Men.
None of them have ability scores in the bestiaries, either, even if there is a character class listed for them.
And that little detail got me noticing something else, something I'd always been aware of but never really thought much about: ability scores don't really do anything during actual play.
Sure, they are used at character creation to cross-reference a bunch of derived stats on their associated tables, stats that help define character capabilities and powers. But the numbers themselves -- say, 12 Str or 9 Cha -- just sat there, taking up space on character sheets that could have been more efficiently used for character portraits, or lists of phat l00t, or whatever.
It's true that successive iterations and editions of the game tried to make the ability scores in themselves useful during game play, leading to varying levels of added complexity. But in the original, they were, functionally, pretty much just place-holders for other things.
Things that were either already accounted for in the stat blocks for non-humanoid creatures, or simply not relevant to their existence in the first place.
And the fact that even the assumed baseline humans of the game could also be designed, through bestiary entries, to run just fine without ability scores solidified my decision: Great & Small would be a grand experiment in seeing whether the world's first roleplaying game and its descendants could run smoothly using "monster" stat blocks alone.
In a way, this is just doubling down on the B/X "race-as-class" concept that originally defined dwarfs, elves, and halflings. In G&S, a character's hit points by level, AC, attack types & damage, and base move rates are defined by their species -- that is, their "race" -- rather than by a separate character class.
I turned the character class concept into "lores," collections of dedicated skills and knowledges that any character could earn levels in. Declaring one of these lores as a "niche" at character creation grants access to special abilities that non-specialists can't get, makes the character better at that lore than non-specialists, and gives bonuses to particular types of saving throw, so class and race have still been somewhat decoupled in the game.
But these niches and lores don't, for the most part, define a character's level-based improvements to combat abilities the way character class did for human PCs in most editions of the original game. They're more like templates attached to a bestiary stat block, to give individual animal PCs a bit of variety. Nonetheless, most members of a given species will be very similar in their overall capabilities, and these species capabilities improve with experience and levels the way an elf's or a halfling's would in B/X.
In case you're wondering: yes, I am going to have the freakish bipedal mutants -- men, dwarfs, elves, and halflings -- defined and played without ability scores, too. The longpaws will be bestiary stat blocks + a niche "template," just like all the other animals in the game.
Because despite their pretensions to the contrary, longpaws are just animals, too.