Without "Arcade" in ArcMap, all text size will always be same, you can not make partial text larger, or smaller
With "Arcade" in ArcPro, I actually can change some part of text size, this is great improvement.
ESRI document is here: https://pro.arcgis.com/en/pro-app/3.1/help/mapping/text/text-formatting-tags.htm
Warning: Arcade only works in ArcPro, when you upload to arcgis online or portal, arcade label will failed
see: https://community.esri.com/t5/arcgis-online-questions/confused-about-arcade-labeling-compatibility/td-p/570695
arcade label format tag
https://pro.arcgis.com/en/pro-app/latest/help/mapping/text/text-formatting-tags.htm