The current research so far has this for an answer, no one has actually confirmed this but the information is reliable enough to believe until some one wants to try it for real.
A bigger bulb should work in a smaller ballast, reports are that some street depts. use bigger bulbs in smaller ballasts because they last longer, so they don't need changed as often. They will burn dimmer, but you should be able to burn 1000 watt bulb in the 400 watt ballast. The reason is the ballast will only deliver 400 watts and won't allow the bulb to pull more than 400 watts thru it.
On the other hand using a 400 watt bulb in a 1000 watt ballast is supposed to cause an explosion. The reason is the 1000 watt ballast will force 1000 watts thru your 400 watt bulb.
You can argue all you want but I've spent days researching this and Bulbs burn different than normal electric theory you are used to applying to other devices. This is the answer until someone tries it.