Larger projects are more likely to have cost blowouts. This is unsurprising, because larger projects have more interdependent elements, any one of which could suffer a setback that flows through to other elements.
Not only are bigger projects more likely to have a cost blowout, but when it happens, that blowout is likely to be larger, both in dollar terms and as a proportion of the project's cost. More than one third of transport overruns since 2001 came from just seven of the largest projects. And there are more and more large projects: 10 years ago the work on hand included four projects valued at $2 billion or more in today's dollars; by the start of 2020, this number had increased to 14.