Do breasts that are more dense weigh more than breasts that are mostly fatty?

Density will increase weight, but perhaps not by all that much.   I recommend an in-office examination as well as a detailed discussion with a surgeon who you are comfortable with.   Finally, make sure your surgeon is a Double-Board Certified Plastic Surgeon certified by the American Board of Plastic Surgery (ABPS), a member of the American Society of Plastic Surgery (ASPS), a member of the American Society of Aesthetic Plastic Surgery (ASAPS), and a fellow of the American College of Surgeons (FACS).Best,Dr. DesaiBeverly Hills Institute for Aesthetic Plastic SurgeryHarvard Educated, Beverly Hills & Miami Beach TrainedDouble Board-Certified, Double Fellowship-Trained Beverly Hills Plastic Surgeon

Yes. There exists a great deal of variability in the density and weight of breast tissue. Younger women tend to have more dense fibroglandular breast tissue which weighs more than fatty tissue. Many older women undergo natural 'fatty replacement' as a normal aging change. This is one reason why our pre-operative examination is so important before a breast reduction procedure. As Plastic Surgeons, we have to assess the type, ammount and quality of the breast tissue to be removed. We are required to make an estimate (in grams) to obtain insurance approval. However, this is only an estimate based upon our experience through years in practice. The actual ammount of tissue removal is always determined at the time of the procedure in order to optimize shape and symmetry.Dr. B 

Although I believe that glandular tissue may weigh more than fatty tissue (not certain however that it's a big difference) my guess is that the difference in the photos you've seen is the actual size of the patient.  It can be difficult to determine whether two patients with similar proportioning are 6' or 5' in torso photos alone.  Thanks for your question.