I found a good quote on the importance of problems for personal growth in Ichak Adizes' book "Managing Corporate Lifecycles":

Each of us is as "big" as the problems we handle and struggle with. "Small" people deal with small problems: the kind of car they own and the quality of their neighbor's kitchen wallpaper. "Big" people struggle with such problems as the quality of their children's education, the environment they will leave behind, and the quality of life in their communities... Addressing and being able to solve bigger and bigger problems means that our strengths and capacities are improving. We need to emancipate ourselves from small problems to free the energy to deal with bigger problems. (p.6).

It's tempting to compare the size of each other's problems :) Size does matter, especially, from somebody else's, i.e. external, point of view. Nevertheless, for pesonal growth marginal size of the problem matters the most. In other words, a positive difference between a new challenge you are willing to take on now and the most difficult problem you solved to-date is be a sign of growth. Living with this difference tends to create Flow, a mental state of full involvement and maximum personal effort.