Answer: When exposed to the stresses of training, the body adapts in a manner consistent with the type of stress applied to it. This is referred to as the SAID Principle (Specific Adaptations to the Imposed Demands). Simply put, you get what you train. Power, strength, endurance and size (hypertrophy) are specific, measurable attributes and the body must be trained in a specific manner to achieve them. As for muscle building, referred to as hypertrophy, a concept called “time under tension” is important. Higher volumes of training (sets) with moderate reps (6-12) gives one sufficient time under tension to cause the structural changes that lead to an increase in the cross-sectional area (size) of muscle fibers. So, yes there is a relationship of weight to reps, as each of the variables impacts the other.
If the weight is very high, then the repetitions performed will certainly be low. In this case the tension is high, but the time is too low. While this is a suitable stress for increased absolute strength, it is not conducive to maximum muscle hypertrophy. Conversely, if a light load is used that allows a high number of reps (15-20), then time is high and tension is insufficient. This type of training is better suited for muscle endurance. The sweet spot is 6-12 reps, done under control, with 75-85% of maximum intensity (weight). So, within the range given, vary the training sessions using lower reps and higher loads in some workouts and higher reps with lower loads in others. Be sure to mix up the exercise you do and feed your body appropriately and you have the recipe for muscle building success.