At first glance these guys could be twins, but if you look a little closer, they are more like brothers.  Kid Action sports darker hair and slightly different face paint, and looks a bit smaller.  Let me point out that Action Boy did have a darker haired variation, so we can't really count the hair color as a difference. 

Taking a close look at the face paint reveals the subtle differences.  Kid Action has more detailed eye and mouth paint, giving him a slightly different looking expression.
 

When you lay them side by side you can see that Action Boy is clearly taller than his reincarnation.  This also gives him a more narrow appearance, although they are in fact the same width across the shoulders. 

Big hands, big gloves.  Action Boy's hand (on the left as you face the pic) is larger and has a wider gripping area.  The thumb is more pronounced and the fingers more angled.  They are also more detailed and more flexible.  This makes them better suited for gripping larger accessories (such as the lower portion of the knife).

Without his boots to give him a lift, Kid Action clocks in at a good quarter inch shorter at the shoulder than Action Boy.  You can see how his torso and legs are deeper set than Action Boy's.  This picture shows the overall similarity in size and detail of the torso, legs and arms.

A side view shows the deeper set upper legs (see the rivet?) and torso of Kid Action.  It also highlights just how close the proportions of the individual parts are.  The height differential is pretty much all due to the mid section.

Take a look at Kid Action's waistline.  The wider waist accounts for the deeper set torso and viola, a shorter boy.  OK, so maybe he his middle has widened and he has lost a little height, but its been 30 years.  Overall I'd say he holds his age pretty well.

I just wanted to add one last shot to show the arms a little better.  You can see the extension is pretty much the same.  In a boxing match neither one would have the reach advantage.

There is probably very little here that you did not already know or suspect.  Playing Mantis has done a fine job with Kid Action, and the only real difference between these figures is in the way the mid section is implemented.  If the height difference bothers you, you can always shim the torso and that should get you to where you want to be.  The slightly smaller hands still accommodate the vintage accessories, and as was pointed out in our review of Kid Action, for this price its just an amazing recreation.  'Nuff Said!