The scale of "large" change depends on your perspective and intent. Usually, any change of significance feels large scale if it dominates a large part of your life. At what point does the PDSA cycle method become less than useful in delivering change?
In healthcare improvement work over the last 10 or more years we have been fixed within the domain of PDSA cycles and the Improvement Model. This is a useful technique in (a) testing out ideas where we are unsure whether they will work and (b) in implementing change on a small scale. These methods have a tendency to produce large number of pilot projects, many of which demonstrate good results. The exhortation is then to "scale up" or "spread" the results to others. So far, this has proven very difficult - and not unreasonably so.
The difference between designing for scale and doing Improvement Model/PDSA type projects lies in the intent. If the intent is for all those for whom the topic is applicable to implement changes to demonstrate an improvement, then consider what might be the opportunities if the focus is on large scale design rather than demonstrator projects:
i) a large scale intent most likely has a large scale context; working with the wider system may help to identify areas where smaller changes can have a big impact. Namely, large scale design does not mean everyone has to do a lot of change. It could mean that if a few, cleverly targeted areas changed, then everyone benefits. We can only find these possibilities if we work with intent and the larger system
ii) we consider how the context interrelates to the aims of the project; context is critical in the process of adopting change ideas. Without highlighting and working on the context change is difficult to achieve
iii) with design as a focus and scale as the objective then we are more likely to seek out innovative solutions to age old problems. Sometimes replicating what we perceive to be good practice may in fact be part of what is hold back large scale improvement.
If you find it difficult to spread from pilot projects then maybe it's time to consider a design process for large scale improvement.
For more on why good practice doesn't spread, go to this book