That's the chief reason, but the cause certainly varies. Swelling (which isn't readily apparent in fats obviously) accompanied by bruising or discoloration can be a sign of acute kidney failure in some cases.  Other times, it may be venous insufficiency (which can be linked to heart issues as it relates to obesity) due to strain or constriction from one or more sources (people that work jobs standing long hours can develop this) or a blood clot in your legs, which is every bit as terrifying as it sounds. Finally, the most common and likely (IMO) cause is a condition called acanthosis nigricans (link is somewhat NSFW but SFL) and is directly linked to obesity, specifically issues associated with endocrinological trouble (the system that produces hormones, surprise, surprise).