I don't entirely get it, why would a troll be only half the cubic size of a horse? I'd imagine they're much taller than a horse, but I don't see where all the extra size for the horse comes from.
I'm not sure what you mean. Presumably, for the same reason that a lion or a donkey is much smaller than a horse, that's just the size they evolved (or were unnaturally selected) to be.
Size in this game roughly correlates to mass, since 1,000 cm
3 (or, for ease of accounting, 1 dm
3) = 1 kg due to how most organic materials have their densities set. Hence, it's likely Toady set animal sizes
1 by looking up average masses.
Wild horses seem to have a size more like 300kg/300dm
3, and so do "light riding horses", but Toady is apparently going for "large riding horses", which can be 500 to 600 kg, while "draft horses" can go as high as 1000 kg. This correlates roughly to an
Andalusian (Spanish) horse. (Meaning the "average" DF horse is actually still half the size of a
clydesdale. This also makes them poor candidates for armored chargers, as those were larger, stronger draft breeds to be able to carry the added weight of armor and an armored rider.)
1 - Note that in DF, "average" size and mass are often biased towards the larger and heavier side of things, because muscle and fat mass added on by stats seem to be added
on top of the average mass, resulting in dwarves, with a supposed 60 dm
3 having an
actual average size of roughly 75 dm
3.
22 - Note that 75kg dwarves are more massive than the "average" human, while dwarves are "too small" to wear clothes fit for a human of 70kg, but can wear 60kg dwarf clothes no matter how massive they get. Clothes are a really weird thing...