How would I go about fixing it?
I installed it and had a look at it to see if it was margins or images. Looks like it's images. You would need to open it up in SkinStudio and edit all the start menu images in an image editor such as Photoshop.
It is a rather nice-looking skin.
Edit- Scratch that. He has set specific image widths on them. So you can open it up in SkinStudio and edit Startmenu / Specific Pieces / Extra Settings Tab / "Width of part" line and set it to "0" or specify a specific value. You might have to dink around with them a little bit.