Martial arts generally, and jiu-jitsu specifically, emphasize that size is not the deciding factor in a fight. A small, well-trained, person can defeat a larger opponent.
However, there is a couple of caveats that are omitted from this overly simplified “truth.”
Size matters, no matter what they say!
The first, and most important caveat, is that a large opponent is always dangerous. It doesn’t matter that I’ve been doing jiu-jitsu for years, someone who outweighs me by 50% is a threat. Whether we are having a friendly rolling session after class or a violent confrontation, I have to acknowledge and respect the size differential. Even a well-intentioned but larger partner is capable of causing injuries just by passing the guard, using knee-on-belly, hitting a single or double-leg takedown, etc.
How big is small?
Before I continue much further down this rabbit hole I am going to quantify size. There are absolute size differences and relative size differences, absolute is “I weigh 10lbs more than you” while relative is “I weigh 10% more than you.” The latter is the more meaningful metric to consider. At lower absolute weights, small differences can still represent meaningful relative differences.
The importance of relative weight shows itself here, with the IBJJF pretty obviously trying to have the max weight about 8% greater than the min weight. For Ultra Heavy and Rooster I’ve used that 8% to calculate what the minimum weight and maximum weight would be.
Tellingly, the same 8% difference results in twice the absolute weight difference between Ultra Heavy and Rooster.
The takeaway from this chart is that the IBJJF regards 8% as significant enough of a weight difference to represent a competitive advantage. In my experience this rings true – someone of equal skill but larger than me by 15lbs will generally win out. Likewise, if I have the 15lb weight advantage I am favored.
Threshold of Caution
From a safety perspective I feel that 20% is the threshold of caution. I’m reluctant to use all of my weight in side control or kesa gatame if I’ve got a 20% advantage. When my partner outweighs me by 20% I am more reluctant to give up side control and more cautious in scrambles.
Once my partner outweighs me by 40% I know that my primary goal is to remain safe while rolling. If they are a blue belt, I recognize that if I’m on the bottom I’m probably going to struggle not to get caught. If they are a white belt I am generally not worried about being submitted, but I have to be super vigilant that they don’t do something unexpectedly and hurt me.
In particular I focus on making sure my arms are never in a position to be americana’d or kimura’d because bigger individuals are capable of exerting dangerous torque from unconventional places. I’m also careful of my ribs, as a 260lb person landing on top of me from side control can easily cause a dislocation – which is a painful injury with a lengthy recovery time.
But what about people who are actually small?
I am not a small person. The people who are bigger than me are big people. The problem with my experiences of risk is that most big people are aware of their size and do their best to roll nicely with their smaller training partners.
From what I have seen, the greatest risk comes to those people who are actually small, say under 125lbs. These people are at greater risk because their threshold of 20% is rolling with a 150lb person… Someone who weighs 150lbs probably is used to being able to use all of their strength and weight since most of their training partners are their size or larger.
If you are one of these undersized individuals, my advice to you is to be extremely choosy in your training partners. New white-belt who weighs 30% more than you? Perhaps look elsewhere.
If you aren’t an undersized individual, remember that size matters and that you don’t have to be a big person to be dangerous to your partner. Take relative weight difference into consideration. Be extra careful when you are putting pressure on your partner’s ribs, whether in passing the guard or from side control/knee-on-belly.