"Think big, start small, scale quickly" has become one of those mantras so widely used that the origin has become impossible to discover (where "impossible" naturally means "takes longer than twenty seconds on google").

"Think big" is easy:  there are plenty of people who like doing it and will churn out ideas at the drop of a hat.

"Start small" is also easy:  there are plenty of people – though largely people different from the first group – who relish making a practical difference on the ground and who will deliver the proof of concept or make the pilot happen.

"Scale quickly" is very hard.  It’s hard partly because the problem is recognised but often wrongly identified.  It is seen as a question of rollout:  how do we plan  and deliver the process for copying the change from one location to dozens or hundreds. The answer to that question is good project management.  The harder question is how to reproduce the conditions which made the pilot successful, and to do so reliably and repeatedly.  The answer to that will include good project management but will need to go far beyond it – it’s about leadership, culture, motivation, rewards and more.

Megan McArdle thinks that the difference between starting small and scaling fast is not well appreciated in the public sector:

You get a pilot program: a curriculum, a teaching method, a high-intensity
preschool program (such as the Perry program) for disadvantaged kids. You do a
rigorous study of that pilot. It produces terrific results. Naturally, we should
roll it out everywhere!

Not so fast. That pilot program has a huge administrative staff whose sole
incentive is to ensure that it is meticulously carried out. In the real
world, that curriculum will be put into place by an administrator whose priority
list is crowded with everything from mollifying the latest lunatic on the school
board, to ensuring that she gets out of town for a three day weekend with her
new boyfriend who she really thinks may be The One.

…

The pilot program has buy in from all participants; schools, teachers or
students who don’t like it, don’t believe in it, or don’t want it anyway, have
already naturally dropped out of the sample. They will thus be striving to
actually put it into place as closely as possible as described in the
prospectus. In the real world 60% of everyone will think this is a moronic idea,
and most of the rest will strenuously resent the intrusion on their
autonomy.

Result: what worked beautifully in pilot will generally fail miserably in
wider execution.

It’s worth reading the longer version.