no one has unlimited resources of course, but this is a sampling question. One has to make choices, and the question was about why you made the choice to reduce your analysis on medium and large size instances.
Indeed, If there are 10 000 small instances and 1 very huge one, why are you choosing to interview the huge one ?
It is impossible to prevent sample bias when planning interviews, but this one seems unnecessary ?
@darius
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.