2 trees grew near one another and have the same growth succession but one is larger than the other. The rate of growth of a tree is based on moisture, sunlight, temperature, and nutritious soil.

Which of the four factors above is the reason for the difference in their sizes? (hint: neither competition for soil nutrients nor moisture appears to be the reason)

Of the remaining 2 factors I felt it was sunlight as the reason. Am I right?

I agree with you. The only major difference must be the sunlight. The smaller tree has probably been shaded by larger trees.