Virtual machine sizes are offered by many cloud IaaS providers as t-shirt sizes. That means the configuration of virtual machines memory, number of virtual processors, and hard disk size is limited.

Most likely they do that for their own benefit. Having a limited number of sizes makes it possible to maximize the utilization of the hosts and reduces what is called cutting waste. A fully utilized host makes the most revenue and enables to reduce costs and be competitive.

Virtual machine size offerings are often named like extra small, small, medium, large, and extra-large. Microsoft initially started with naming their virtual machine sizes like this, but later realized this naming has its limits. What about a super-super-large virtual machine name. So, Microsoft switched to names like A5, A6, A7, and so on.

The disadvantage of t-shirt sized virtual machines is that you are probably going to pay for resources you are not going to consume...