OK, so a 55 gn (grain) X193 needs to be flying at 2,800 fps to fragment.  The 77 gn bullet is even heavier, so it is going slower.  So how does the 77 gn and the lesser gn bullets (like the 72 gn) become so effective?

One has an open tip, the other does not.
One may have a softer or thinner jacket, the other may not.

The velocity needed for a projectile to expand/fragment is dictated largely by the construction and design of said projectile. The 193 just happens to have a higher velocity threshold than many alternatives. The 77 OTM isn’t up on some pedestal, there are many others that perform considerably lower.

Science behind the jacket that you can't see.  From the outside, all these different bullets look the same, right?  They aren't.
I've been through Sierra factory, and seeing the process first hand was enlightening.  
If you're ever in central Missouri, it's worth the time.
Sierra bullet jackets

FWIW.

If you ever have an opportunity to go to SCI national or Dallas SCI annual convention.
Ask the African Professional Hunters that exhibit there, "Light & fast, or heavy & slow?"
The term is Heavy For Caliber.  

American's became speed junkies in the 50's by Mr Roy Weatherby.

Pistols put holes in people, rifles put holes through people, and shotguns spread hamburger all over the wall.
Physiology dictates there 2 ways to terminate life, pulmonary and electrical.
Pulmonary is the most common choice, and 2 holes are better than 1.

Not all bullets are created equal

M193 at +2700 fps does not fragment upon impact.  It penetrates, and like most bullets, starts to tumble.  When it hits roughly 90 degrees, pressure (usually) causes the bullet to fragment.  Remember, this is an FMJ round that is not meant to fragment, but push it fast enough, once it gets sideways, it blows apart.

Mk262 type rounds are built differently.  Thinner jacket, open tip, LONGER bullet.    Again, when it it hits, its ass heavy structure causes it to tumble.  Longer bullet, thinner jacket, different construction = lower fragmentation threshold.

As stated

1. Thinner match jacket and soft lead construction. The jacket helps hold the soft lead together upon yaw. When it is weaker, the bullet holds together less. The lead alloy in a match bullet has lower antimony, tin, and other alloy levels that might make it harder to fragment.
2. Long bullet means easier to break and more forces acting upon the ends diring yaw. Think of a short stick vs a long stick of same diameter, trying to break both over your knee. The added leverage of the longer stick makes snapping it in half easier.
3. The length and air pocket at the tip of the bullet makes for a less stable bullet. While a 55 grain bullet is stable with a 1/12 twist and 62 grain SS109 is stable with a 1/9 twist, the 77 SMK requires a 1/8 twist to stabilize. The greater instability is exagerated when a bullet tries to fly through a water based substance (tissue) which is much more dense... the twist is inadequate for stabilization and the bullet yaws. The open tip is a byproduct of production... it does not do anything like a HP pistol bullet does, but the air pocket shifts the center of gravity to the rear and enhances yaw.

The faster yaw of a long, thin jacketed, soft lead bullet,  results in a shorter neck till yaw, lower threshold velocity for yaw based fragmentation, and greater fragmentation overall, when compared to M193.

Change the jacket thickness/hardness, lead composition, bullet design (air ppcket at tip), etc. and you can remove the low velocity fragmentation characteristics. 75 grain Wolf HP, for example,  having a steel jacket, does not fragment like 77 SMK or Hornady 75 OTM.

Also the heavy 77g bullet will still be  faster at a distance then the 55g and 62 bullets.   At 100  yards m855s velocity is already slower  than the mk262 77g at 100 yards.  The lighter bullets come out faster but loose velocity quickly.

A lot of people lose sight of this.  15 years ago when Ammo Oracle was published, the 55 FMJ was the clear better choice.  15 years ago the typical barrel was 20" , or at the least, 16".  Even with that, once you get past 100 yards, that magic starts fading fast.  

In the now more common 10.5-16" variants, and 20" not really being the standard length anymore, that magic 2700 number is closer to 50 yards, if at all; depending.  And once you get down to that level, almost all other bullet designs are going to be more effective on target.