Simple Pulse Qualities: Width
A thin pulse suggests a lack of substance in the vessels. As the graphic below describes, this is most likely a deficiency of blood or yin. However, dampness accumulation can also obstruct the quantity of substance in the blood vessel, and in the case of an overweight person, you might consider dampness as causing the thin pulse too. This is described in further detail on the thin pulse page.
The wide pulse arises from an excessive pathogenic factor or excessive constitution (even if this means only youth) fighting off the pathogenic factor.
Next: pulse length.