reference architecture listing small, medium and large is associated with requirement of an organization in terms of number of catalog items to manage or number of concurrent deployments needed
like the screenshot taken from the same guide you are referring. (Accordingly medium or large deployment reference architecture is also mentioned)
now if you refer instllation guide @ https://docs.vmware.com/en/vRealize-Automation/7.5/vra-75-install-upgrade.pdf ,
you will get to know that Minimum deployment is a very simple deployment of vRA which I would like to do in my lab environment with one vRA appliance and one IaaS node with all the IaaS services like IaaS Web and IaaS manager and Dem Orchestartor / Workers etc installed. (I have not done this in a long time)
but when it comes to Enterprise deployment, it's about getting IaaS Web and IaaS Manager services to be separated and getting the whole deployment done in a distributed manner so it becomes easy for you to scale out later. Like to begin with you did follow small architecture based deployment, but later you have a requirement to go to Medium or Large, you can do so.
Minimum deployment cannot be converted to Enterprise(distributed) later on.