The Alshich HaKadosh explains this in his commentary on Shir HaShirim. He states that when a person performs a mitzva, the source of that mitzva becomes activated and is filled with light, which also lights up a person's neshama, all the way down to his soul on earth, and even his body. The more kavana he has, the more light. However, Torah study creates a might greater spiritual light compared to the performance of any other mitzva. To explain this, he gives a mashal.
Torah study, and mitzvot, can be compared to a tree trunk and its branches. The branches get their nourishment from the trunk, and the trunks gets its nourishment from the soil, via its roots. So too, the light of a mitzva gets its has its source in the Torah, which embraces all the mitzvot. When one improves the tree's condition, the branches also gain. Similarly, when one studies Torah, he brings light to the branches (i.e. the mitzvot) as well. However, if one simply tends to a single branch of the tree, only that branch benefits, not the rest of the tree. Remember, the mitzvot get their light from the Torah, but just that branch is lit up in that case.
The Alshich then goes on to explain how this is not just talking about studying the secrets of Torah, but even the simple meaning lights up the whole tree. The best way is to study the simple meaning, but have the symbolic, homiletic and hidden meanings in mind, as well as channel the words through the prophets and writings, and they will bring a very great light into the world.
The goal of all Torah is for Hashem to become known1. Studying the Torah, His revelation, and which encompasses the knowledge all of His Mitzvot (and studying them is equal to performing them on many levels2), is the best way to get to know Him, and produces more light in this regard than the performance of an individual mitzva.*
*Of course, other answers have pointed out how studying a mitzva is not a total replacement for performing them, and in a very important respect, the action is greater than the study3. This is just a side point because we are comparing like for like here, the performance of the "study of Torah" mitzva vs. the rest.
1 - e.g. אתה הראת לדעת in davening
2 - I will look up the source for this shortly
3 - המעשה הוא העיקר