For anyone who needs a higher spec machine. It turns out you need to request a quota increase for Fargate on AWS to use machines with larger than 8GB of RAM and 2 vCPUs. At the time of writing, there is no option for an increase in vCPU/RAM, but if you request a quota increase for something under Fargate, and explain the issue your encountering, the support team seems aware of the issue and are able to help you resolve it.