The platform makes API calls to Azure to get other sizes from this API Resource Skus - List - REST API (Azure Compute) and make the appropriate recommendation based on the VM metrics. There is not any logic in place to restrict the recommendation to the existing chipset. The only time recommendations are restricted is when the machine is burstable. Otherwise, the platform will make recommendations to change chipsets if that is what will save the most money.
More info on the New Azure Rightsizing tool