Broadness shouldn't imply shortness, according to the raws:
These tags establish the growth phases of the creature's life. The format is (BODY_SIZE|<year>|<day>|<average size>).
[BODY_SIZE:0:0:3000]
[BODY_SIZE:1:168:15000]
[BODY_SIZE:12:0:60000]
These body modifiers give individual dwarves different characteristics. In the case of HEIGHT, BROADNESS and LENGTH, the modifier is also a percentage change to the BODY_SIZE of the individual creature. The seven numbers afterward give a distribution of ranges. Each interval has an equal chance of occurring.
[BODY_APPEARANCE_MODIFIER:HEIGHT:75:95:98:100:102:105:125]
[APP_MOD_IMPORTANCE:500]
[BODY_APPEARANCE_MODIFIER:BROADNESS:75:95:98:100:102:105:125]
[APP_MOD_IMPORTANCE:500]
...you know, I think I was reading those numbers wrong...I was taking it as 1/7 chance of getting each of those specific percentages, but reading the raws again, they talk about intervals, not values. So perhaps it's "roll an interval (equal chance of each of the six intervals), then roll an equal distribution random number within that interval to get the actual size modifier".
That would make dwarves normally much closer to the average size than I'd been thinking.
There are also similar modifiers for each individual body part, separate from the ones for the whole body that I quoted, and some of those are named "HEIGHT", "BROADNESS", or "LENGTH"...if those also have an impact by modifying the weight of that body part, that would also tend to make the body size stay a bit closer to the average.