To put this into perspective, the SS109 design, which both Canada and the USA adopted, was optimized for a 20" barrel.
The bullet was not designed to fragment. Despite the legalistic stance by the Yanks (whose lawyers - how I envy their generals - know who they are working for), bullets specifically designed to fragment would almost certainly be ruled to be in violation of the Conventions in a court of law. The SS109 was designed as a semi-armour piercing round, intended to defeat the at-the-time most-probable Warsaw Pact threat with helmet and possible body armour.
The formula for ballistic stability is, shall we say, 'impressive', with about a dozen variables in it. Key ones WRT this discussion include velocity, bullet diameter, rate of spin and the density of the medium through which a projectile is moving.
The SS109 bullet is essentially stable in air (ignoring nutation and such). Above a certain speed, however, when it transitions into a much higher-density medium (eg a body), it becomes unstable. It wobbles and tends to tumble. (Curiously, if it were flying butt-end first, it would be more stable at average velocities.)
The construction of the bullet, quite adequate to hold it together in flight, is very often not sufficient to withstand the stresses the tumbling puts it through, especially under the exceptionally high rate of spin a 5.56mm bullet demands. Given the relatively thin jacket possible in a small-diameter projectile, the bullet cannot hold together and usually fragments in what some describe as an explosive fashion. The wound channel is larger and fragments tend to go off in all directions, giving it both high lethality and high stopping power.
Once however the in-air velocity of the bullet drops below a given level, the bullet will remain stable even when transitioning into a water-based medium (body). While it will produce a very nasty wound, it generally will not tumble or fragment and hence is usually possessed of a much lower stopping power, even if the wound is eventually lethal.
Under normal conditions, including those applying in Afghanistan and Iraq, that velocity from a 20" barrel obtains out to about 200-250 metres. Put the same cartridge through a much shorter barrel (eg C8 or M4) and that key distance drops dramatically, sometimes to as little as 75-100 metres.
(I acknowledge the weasel-words involved - 'usually', 'most often', etc. Bullets do the damnest things and it's not always predictable. It's a good way to put your money, however.)
The SS109 bullet is not a bad compromise when used in the barrels for which it was designed. When paired up however with the current desire for short, more handy barrels, accuracy is inherently no worse, but the stopping power down-range drops off rather dramatically.
To clarify the rate of spin question, projectile diameter is key in ballistic stability. The higher the diameter, the lower the RPM needed to provide gyroscopic stability, ie that stability provided by spin. (Think about how easy it is to spin a short, wide child's top. Now think about trying to spin a pencil on its point... Assuming the pencil holds together under the spin, it's possible but the RPM required is considerable.)
The L15 155mm projectile, the C21 7.62mm ball bullet and the SS109 all have about the same shape and move at much the same velocity. (Yes, there are differences, but wait to see where I'm going with this.) The point is that, almost entirely due to their diameter, the L15 needs to be spun at about 16,500 RPM to be ballistically stable, the C21 at about 165,000 and the SS109 bullet in the 300,000 range. Once the projectile gets out of true WRT its line of flight, that very high spin rate makes it very hard to hold everything together.
Bottom line - hitting is good. Hitting is essential. Hitting is that without which everything else becomes pointless. But the smaller the bullet, the more you need velocity to maintain stopping power. Trading off a few inches of barrel length, convenient in so many ways though it is, lowers your velocity considerably.
I leave it to those actually on the ground to decide the question of fewer big bullets vs more small bullets. As I see it, there's no perfect solution.