ive vegged under a 1kw as well, my first grow, and have found no difference in size whether my 1kw or 400w, plants in veg require far less light to mature than flower.
sur
the only way to double size is perfect conditions in a fogponics type setup, i grow in soil, and my plants dont see a difference in the light. Sure, if i was going to veg alot of plants that required a light with a larger foot print then i would go with my 1kw, but i only veg 9 plants at a time in solo cups, i can veg for two months in solo cups before they become rootbound. Try a smaller light with smaller pots, you will see that they didnt need or utilize as much light as you thought.
deminishing return is a bitch, and sometimes we throw more light than a plant is capable of using without increasing nutes/co2 ppm, (only way to increase speed, or hydro)
Natural sunlight is about 10,000 lumens, and plants dont use all of it, so with a 1k, it wouldnt make sense, unless you had enough plants to cover a 4x4 area.