By default, changing the text of a node does not change the node's size in any way. Therefore, I assume you have activated option "Dynamically Adjust Node Size to Label Size" (see "File" -> "Preferences", tab "Editor"). This feature is essentially an automatically started instance of "Tools" -> "Fit Node to Label" and also uses the settings chosen for said tool. You probably have said tools "Adapt to Maximum Node" enabled. Either turn "Adapt to Maximum Node" off or turn "Dynamically Adjust Node Size to Label Size" off altogether.