The "beauty" of strength training is that depending on your build you are bound to have some lifts in which you are predisposed to be stronger or weaker. Very few guys can say that they have perfectly balanced strength levels, unless they worked very hard to correct biomechanical disadvantages.
For example take two opposite body types:
a) short limbs/relatively long torso ... in Soviet sport-science literature this is called a "dolicomorph" but in layman's terms we could say "short and stocky".
b) long limbs/relatively short torso ... still in Soviet sport-science lit this is called a "brachiomorph"
Now, the first body-type is built to be mechanically advantaged in lifts such as the bench press and squats while the second body type is mechanically advantaged for deadlift-types of exercises, olympic lifts and most pulling movements.
So in that case a lighter/shorter dolicomorph might be able to bench much more than a heavier/taller brachiomorph. So in that regard relative strength (strength vs. bodyweight) might not be all that relevant.
However, if we take two individuals with the same body-type, the heavier lifter (all other things being equal) will always have the advantage. First because the added muscle mass increases strength potential, second because in some lifts the added "bulk" actually provides a certain mechanical advantages, often by reducing the movement's range of motion. Take for example a guy with a huge chest, he will have to lower the bar to a lesser extent because of his muscle mass. Same thing goes for a guy with huge glutes, hams and calves when we are talking about squatting. This is why, in powerlifting/pure strength training, added weight, even in the form of fat can help you lift more weight. So an heavier lifter should be able to lift more (given a similar body type and level of training). Hence the reason why a 180lbs guy bench pressing 500lbs is more impressive than a 300lbs guy doing the same weight.
The only basic lift where added bulk doesn't provide much of an advantage is the deadlift because being "thicker" sometimes prevent the lifter from keeping the bar close to his body, which increases the difficulty of the lift.
As it was mentionned earlier in the thread, this is the reason why strength sports like powerlifting and olympic lifting use formulas to adjust the weight lifted to the body weight, so that lifters can be compared.
That having been said, as I mentionned earlier... being bigger, even in the form of muscle mass, is no garantee of being potentially stronger because different body types are built to perform well on different lifts. Take me for example, I'm a short and stocky fellow... as a result I have always had great facility squatting and bench pressing big weights. However the deadlift has always been a huge achilles tendon of mine. This is a shame because I actually started out as an olympic lifter (a sport which requires a pulling-advantaged body). When I trained with several members of the Canadian National team at the Centre Claude-Robillard I would always kick everybody's ass in the squat and front squat... including several guys who competed at the world championships, Pan Am games and even Olympics. However when it came to performing the actual competition lifts I wasn't even remotely close.
That's not to say that it's impossible to go against your natural leverage. One of my best friend is a natural powerlifter/strongman competitor. He's 6'3" and around 290 with super long arms and legs and a very short torso. As a result he is a natural deadlifting machine. Heck when he was 16 years of age... at a bodyweight of 178lbs (still on 6'3") he held the Canadian junior record with a 600lbs effort!!! However for the longest time his bench press and squat sucked.
But he kept on training hard and eating everything in sight and now at 29 years of age he bench pressed 535lbs in competition (making 575lbs in the gym) and squatting in the low 700s. It took him a long time, but he was able to build decent lifts against his poor leverages.
Bottom line is that added size will help you lift bigger weights, however sheer bodyweight is not the only factor to consider when comparing two lifters.
