What are the forces of growth that have made the science of fusion evolve into the research project ITER, and how may they be accounted for with regards to the particular challenges to which the project has been subjected during the last two decades? This problem is intended to shed some light on the underlying causes for the difficulties faced in modern fusion science. Currently, ITER is the largest venture ever devised in fusion and the very pinnacle of over 60 years of global research. Its primary objective is the scientific and technical demonstration of fusion as a practicable method for future large-scale energy production. Despite several plus points, developing fusion through ITER for energy purposes is not devoid of complications; if truth be told, its benefits are more or less offset by its challenges. I maintain that the source of these challenges is that ITER has become too costly, and will possibly remain so regardless of how its challenges are dealt with. Indeed, the cornerstone of the excessive expenditures is the project’s accumulated size which is primarily dictated by the particular reactor technology it utilises, called tokamak. Paradoxically, this very size is what makes the project push forward. Likely explanations for this situation are found in a literature study on fusion research history. This is conducted firstly by defining the history in terms of three variables: the international fusion community, the political framework, and the fusion reactor technology itself. Secondly, these are organised into four decisive phases and then framed, through theoretical discussions concerning the concept of Big Science, as inherent mechanisms of growth; mechanisms that have collectively facilitated the scientific progress in fusion. Finally, the actual functions and interrelatedness of these mechanisms are clarified through perspectives on path dependency, deriving from evolutionary economics. The objective here is to demonstrate that the evolution of science can apply well to growth patterns that are comparable to those of markets and large technological systems – indicating science as a concrete system of its own. Accordingly, I am suggesting that fusion research is a system of science that has culminated in the ITER project after years of accruing improvements and momentum. Ironically, this momentum has made ITER become detrimentally large and inflexible over time owing to its profound contingency on the high-demanding tokamak technology.