As you already considered, high value resistors are usually not rated for it's full power. Also true for some low value resistors.
Usually resistor ratings show power, voltage and some current ratings, mainly packaged limited. A certain package has some power dissipation capability considering it's size, which limits the power, also a certain distance between the pins, limiting the voltage and a certain lead thickness limiting the current to maximum rates.
After that, to stay within specs you want a maximum temperature rise, limiting the power even further, a certain linearity, limiting voltage as well and the internal elements have a current rating that could also be a limiting factor.
Current is the one that could not be present as the range of resistors goes so much low and power might hit the limit first for the lowest value of the product. Power and voltage are usually there, certainly for a 10M resistor voltage is the limiting factor so don't trust the power to have any significance there.
JS