The legend shrinks when created with no legend title and the following conditions are present:
- The legend title option 'Show' is checked.
- The map contains one or more layers set with a scale range.
- The map is zoomed into while in Data view.
Uncheck the 'Show' option in Legend properties before zooming in. If the data has already been zoomed into and the legend is broken, delete and reinsert the legend with 'Show' unchecked.