The text boxes on some of the slide templates in PowerPoint auto resize when you type in them. If you continue to type something after you have reached the bottom of the text box, the text gets smaller so that all of the text fits into the text box.

If you symbolise text in a text box which is set to auto resize the text, and you have chosen the symbolise above/below paragraph option or the align to words option, then the text auto resizing is automatically turned off. This is by design - this is to prevent you ending up with really small text when symbols are added.