The interactive routing always use the via size set up by the rules, if no rule is set up then it would default to a system default size of 50 diameter / 28 hole size.
You can access this by going to: Design ► Rules ► Design Rules ► Interactive Routing ► Routing Via Style.
Make sure that a Routing Via Style rule with the desired size is created and enabled the Routing Via Style rule in the PCB Rules and Constraints Editor.