Well, in, say, melee in SR, Agility and skill, naturally, both come into play. To me, it makes perfect sense that the Body 4, Strength 4 dude with a 6 Agility and 5(+2) Martial Arts(with some + Martial Arts qualities), should be able to beat the modified 12 Body, 14 Strength, 3 Agility, 1 Unarmed Combat troll's ass. Sure the troll is three times his strength, but the other guy is rolling tons more in skill dice(the troll tossing a max of 5 if he uses Reach, the other guy 13. Not to mention Strength 4 guy might have +2 DV, hardliners, and maybe even some aluminum bone lacing giving him a DV equal to the Troll.) Of course, this takes skill into account too.
In the boxer examples, they very well might have equal skill-but Id be willing to bet the smaller guy might have a couple more Agility tacked on.
Hmm...let's say in SR, the 'lightweight' is Body 3, Agility 5, Reaction 4, Strength 3.
The 'Heavyweight' is Body 5, Agility 3, Reaction 3, Strength 5.
Both have stats above average-3 is still a little above the norm. The big guy isn't slow; the small guy isn't weak or flimsy(even those lightweights can no doubt hurt with a punch).
Both could have 2 levels of Boxing for +2 DV, Hardliners(for the hell of it), for +1 DV. Both have skills of 4(+2). So Lightweight is rolling 11 dice, with a DV of 5(2+2+1). He gets his 8 Blocking dice, and say the fast guy has a Dodge(Melee) of 2(+2), since he's a little faster. Big guy has 1(+2). So the little fellow has 12 dice on Full Defense, which will help his lower Body(he rolls two less dice for defense).
The big guy rolls 9 dice to attack(not bad at all), with a DV of 6. He gets 7 dice on defense(maybe a little tiny bit harder for him to get out of the way), 10 on Full, again, a little less but not bad.
To me, it seems about even. Big guy has a better chance of soaking the hits and dishes out a bit more-the little guy has a better chance of dodging, and dished out a little less. There may be a factor or factors i'm missing though.
I could do a Fighting Game crossover, but those aren't realistic at all(i love em anyway, though.

). Nowadays you have the whole 'Priority' thing which is really big-a character with crappy stamina, crappy power and just plain speed with mad priority is usually going to have an advantage over a character with mad speed, mad damage, good stamina but crappy, last-ranked priority. Some guys have speed but not strength, some vice-versa(and many times in fighting games the glacier-types are the ones that suffer many times), the Glass Cannons(the ones with lots of speed and strength, and even good priority-but really crap stamina.) And so on.
I kinda want to make a bunch of sample guys up(different fighter types) for the hell of it, but I dunno, it may just be my Vappu-hangover talking.