Now: Keep in mind, as usual, that I interpret the distances in inches literally on the tabletop, as given in Original D&D Vol-1 and Vol-2 (Men and Monsters); and assuming the same scale as the figures used, so about 1" = 5 feet. The prescription that appeared in Vol-3, that 1" = 10 feet, I ignore. (See the sidebar on house rules for more discussion.) The figures and counters that I used are at the standard 3/4" (or 20mm) square base for men; or 1×2" (25×50mm) for horses.
Also, my general assumption is that there is some spread between figures; that is, a few feet between ranks of soldiers, or some amount of scattering in the face of melee/ missiles/ magic fire. Here's what that "sparse" crowding looks like:
|Areas of Effect: Sparse Crowding|
As you can see, a fireball catches about 7 men; the red dragon breath cone catches about 14 men, the green dragon cloud a similar 15, but the blue dragon's line is smaller with only 7 men encompassed (further reduced for lesser types like black and white dragons). For cavalry, the numbers would technically be reduced (3, 5, or 8 figures hit by breath). But for simplicity in Book of War, I rounded off and said that all of these attack forms hit about 10 men, that is, 1 figure at mass scale.
However, perhaps you don't share my assumption about men being spread out a bit in most cases. If the figures are packed maximally "dense", then here's what you get instead:
|Areas of Effect: Dense Crowding|
In this case, a fireball hits about 13 men; and the breath from red dragon hits 26, green dragon 30, blue dragon 13. Here we find 5, 8, or 12 cavalry figures in a dragon's breath area. In principle, this might argue for the red and green dragon breath types to hit up to 3 figures of men at mass scale; but all of the others (like fireball) still round off to 10 men, so I think that gives extra evidence for the simplicity of the Book of War rule that only 1 mass figure can be affected.
More generally, I find that keeping this in mind helps to adjudicate at any time we're abstracting the action away from actual figures on a tabletop (for example, when using random/abstract opponent selection as in the AD&D DMG).