would say that the limited sunlight is probably why it has not grown more but there could be other things contributing to it like soil and amount of nutrients availble to the plant, would probably try transplanting to a spot where it receives more sunlight just be carefull digging it up and try not to disturb the roots to much you may want to plant it slightly deeper if the stem seems weak you can bury to just below the first set of leaves.
you might want to pick up some superthrive or other fertilizer that helps plants bounce back from transplanting and reduce transplant shock.
its really to late to be topping as it will start flowering very soon and would probably be better off just tranplanting it for now.