Hi Simon,
The theme uses the superfish js menu with supersubs. (http://users.tpg.com.au/j_birch/plugins/superfish/)
What supersubs does is try to size the dropdowns to fit largest items within parameters of min and max widths (given in em units). There will always be slight variations in this due to browser rounding differences and font sizes (since it uses em units). The theme is also responsive so this will also change for different browser window sizes. But the menus are designed to wrap when they exceed the max width (currently 27em).
To adjust the parameters for this you could edit /js/tricks.js. The parameters are clearly identified at the top of this file. In this case you could try experimenting with the extrawidth value or just increase the maxwidth to accommodate your menu items.