It would help us greatly if you could post a photo of this phenomenon.
I think you may be describing peripheral edema, which is an accumulation of fluids extravascularly (outside blood vessels), This can result from lack of circulatory proteins in the blood, or a back up of fluids due to a circulatory problem from heart or other organic disease.
Since you asked for a cause I am providing a list of causes from the veterinary textbook: "Veterinary Five Minute Consult" as follows:
Low Oncotic Pressure (Hypoalbuminemia) (low protein in the blood)
• Hepatic disease
• Protein-losing renal (i.e., nephrotic syndrome) or gastrointestinal disease
• Severe blood loss with noncolloid fluid replacement
• High hydrostatic pressure
• Right-sided or biventricular congestive heart failure (CHF)
• Pericardial effusion with tamponade
• Right atrial mass
• Vena cava obstruction
• Overhydration, especially if associated with acute oliguric renal failure or hypoproteinemia
High Capillary Permeability
• Vasculitis (e.g., infectious disease and tick-related disease)
• Some snake venoms
• Trauma, burns
Lymphatic Abnormality (i.e., Lymphedema)
• Infiltration or abcessation of regional or multiple lymph nodes
• Traumatic lymphatic disruption
• Congenital malformation of lymphatic system
It would be best to take your dog to the vet to have the problem evaluated. Since I can't see the problem it may not even be peripheral edema. If could be enlarged lymph nodes, or fat pads.