When you have a lighter weight ground engaging implement, then start adding weight, the implement itself will eventually fail either at the welds or the metal itself bending or breaking.
A 500-600lb unit is no where as strong as a 1000-1200lb unit. But you add 400lbs to it and you are asking it to be.
So while the light weight unit with added weight may very well work as well as a true heavier unit, it will not last any extended length of time being used in that way. If you have extreme conditions, the light weight unit could very well fail immediately simply because it does not have the structural strength to withstand the pressures exerted by having the additional weight added to the unit.
That is why you don't just add weight.
Lot's of people do it all the time without any negative results, there are always exceptions, most that are ok don't have severe conditions that they are dealing with though.