My experience from the inside of districts (admittedly mostly in a state with relatively generous parental due process rights) has been that larger districts are more likely to be in compliance with the law (and hence to offer services), while small, UMC districts are highly-variable. Quite often, I've been in meetings where parents of students transferring into our district from a smaller, superficially more highly-regarded district are relieved to find they will not have to fight us tooth and nail to obtain appropriate services for their child. A large district I worked for at one time had a huge range of services, far superior to those available in any of the surrounding suburbs. OTOH, one of my past districts was a high SES/highly-ranked district, and was also quite responsive to parental concerns.
I suspect that the small districts are prone to effects from 1) a smaller, more homogeneous student population, and thus less experience with low-incidence or high-needs students; and 2) individual district representatives and their personal biases and skill sets/deficits, and thus more extreme effects from personnel changes. Large districts are forced to have better-articulated policies and systems, which means parents usually have a clearer path for going up the ladder, and less inconsistency moving from one staff person to another, and are also more likely to have a pool of low-incidence students, which means they may actually find it cost-effective to employ specialists and design programs for those students' needs.
