Blender UV Unwrap - Face Not Mapping Correctly

Hi guys.

I marked the face of a cuboid as a seam, and then unwrapped it. However, it shows up like the image below:

Does anyone know why the face is so thin? Is it because I used edit mode to stretch the object? And, more importantly, does anyone know a solution?

Thanks in advance for the help,

Binkadin.

Unapplied object scale. Object mode, ctrl+A -> scale. Unwrap again.