I'm getting into my fitness and am beginning to see an increase in muscle mass. However, I look at people like Bruce Lee, Elliott Hulse, Mohammad Ali and Wim Hof who are capable of incredible strength but do not look as "buff" as people like Arnold Swarzenegger. It is clear that an increase in muscle mass does not necessarily lead to an increase in strength.
How is training for strength different to training for mass?
To add a new angle to this question, is there a correlation between the two?