I doubt Tolkien ever bothered to think about that...but if if this a truly crazy question I'll try to answer.
In note 11 to this chapter, Tolkien remarks:
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The tales, such as The Faithful Stone, that speak of their transferring part of their 'powers' to their artefacts, remind one in miniature of Sauron's transference of power to the foundations of the Barad-dûr and to the Ruling Ring
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As such, I believe that we should compare the watch-stone to the One Ring so as to get a better idea of how things worked.
Now surely, a drûg could only create a certain number of watch-stones, since he did not have limitless power. Same goes with Sauron. This is why he couldn't create any other Ruling Ring, there was not enough power left all in all.
And so I also expect that the strength of a watchstone would depend on how much power wa given to it and just like that that for a larger stone more power was needed, this seems rather normal.
However, I have no idea about watch-stones of different sizes having the same strength when given the same power and I do nto wish to speculate on it because I find this is a bit too far off from Tolkien's world.