Text in text box is narrowed when width of text box reduced.

[MC147]

I have a strange problem with text boxes in Word since moving to Office 365 from Office 2007.

I have some text in a text box in a Word document. When I reduce the width of the text box, I was expecting the text to be re-justified - i.e. spread the text over more lines. This is what it did in Word 2007. However, it did not do this in Word 365 - it made the text narrower, still on the same number of lines.
Is there some setting in Word or the Text Box that specifies this behaviour. It seems strange to me not to re-justify.