Larger reactors may simplify the plasma physics, but it complicates the materials engineering significantly. A huge problem already is creating a structure that can bear the weight of the reaction vessels and the gigantic magnetic fields used for containing the plasma, and also continue to do so for a decent amount of time after being exposed to the constant neutron bombardment of D-T fusion.

I very much doubt currently known materials and structures could be used to construct a reactor 10 times the size of ITER.