With Mole, the heat build up would be excessive if you put a 1k in a 650 fixture. The 650 bulb is hot enough; they had to redesign the tweenie because of it. The Mole bulb sockets are quite different for the two bulbs. The 1k bulb is 3 times as tall as the 650 bulb. I would imagine the same is true for Arri.
Even if you could squeeze the higher wattage bulb into the 650 fixture, the taller bulb would mean that the fillament would no longer be properly aligned with the center of the rear reflector or the fresnel lens, making for a much less efficient light. This should not be confused with the weaker bulb that is available to fit into the 1k housing. I believe it is a 750w bulb but it is the same size and shape as the 1k bulb so that it still aligns properly. Can't recall the three-letter name at the moment, but it should all be in the Arri or Mole catalogs.
Short answer, spend te money on the bigger units. It's always better to put a weaker bulb into a larger housing than the reverse.