Soccer cleats do not come in widths, which can make it difficult to obtain a good fit. Since many medium width cleats will run either wide or narrow, you can use a side-by-side comparison to identify volume differences between different pairs of shoes.
The two pairs of cleats below illustrate a narrow lasted cleat on the left, Adidas Predator Absolion TRX , and a wide lasted cleat, Nike Mecurial Victory II, on the right. The differences in width are highlighted.
Comparing the uppers, notice how the throatline (opening) of the Nike is so much wider than the Adidas. Also notice the difference in toebox shape and width. The Adidas is more tapered around the toes and the Nike is more rounded, accommodating a wider forefoot.
In evaluating the lower, you can see how much wider the forefoot and waist are in the Nike than the Adidas as well. If you have been having difficulty finding the perfect fit, using this method should help. These principles also apply to other shoes (tennis, basketball, football, etc.) that only come in medium widths.