Darpa started off small and informal. For years it was less than ten people.
Starting too big often causes heavy process. People always spend money for a purpose so the more money there is up front, the more expectations there are attached to it. One failure mode is that the money factory (Innovation orgs need a money factory) doesn’t trust a low-track record organization and requires heavy process to create assurance that the money will be well-spent. Restrictions on spending money happen when you reach trust limits.
Starting large makes opacity hard, and Opacity is important to DARPA’s outlier success. Starting large with large expectations and scrutiny makes it tough to execute on things that seem stupid and encourages organizations to work on things that seem sexy which makes it hard to generate true outlier output. If you know it’s easy to shut something down it’s easier to start.
Culture and trust takes time to build and if you haven’t spent years building up an organization so if you start big without heavy process it will just lead to a shitshow. As far as I can tell, this point and the previous one is roughly what happened with Google ATAP.
Regardless of size, directly copying all of DARPA’s processes leads nowhere good. Many of the processes were built up over years to fit DARPA’s exact situation, and as noted earlier, you should Pay attention to DARPA’s informal process and ignore formal process.