The limiting factor for ITER was the strength of magnetic fields that could be attained from the superconductors available at the time. The promise for ARC/SPARC at MIT is that the field strengths available have recently doubled, and performance goes something like the 4th power of field strength, so a smaller reactor can reach the conditions that only a larger one could have reached when ITER was designed.

These smaller reactors should be cheaper and faster to build.