Assuming that "medium" is logarithmic, i.e. about 1000 or so users, there are a few hundreds of these - so you presumably intend to subsample these medium-sized instances, e.g. aim at 10-20 interviews.
Splitting up into e.g. 5-10 medium and 5-10 small instances would increase the sample error and bias, as @jcolomb said, but also increase the chance of better studying all scales: big + medium + small.
The risk of the big crushing the small seems like a significant governance question.
@boud @darius @jcolomb I would also argue that a ‘medium and large’ runs a significant risk of skewing heavily white, and mostly male.
Plus the fact that it’s folks from the US, with US money, on a network that for once isn’t as dominated by American interests and governance as almost everything that came before.