Dear 100 Hour Board,
How much do you think the vampires from Stephenie Meyer's world weigh? From what we know, they are:
*Hard as marble
*Sparkly like diamonds
*Loud as crashing boulders when they slam into each other
*Silent and graceful when they move
*Don't leave footprints (at least on the forest floor in book 2).
The only clues I can find for weight is that Renesmee (who is half vampire) is said to be "sturdy" when Charlie holds her and Edward doesn't break Bella's old rocking chair when sitting on it. Thoughts or ideas?
You could be silent and graceful when you move even if you were really heavy - it would just require stronger stabilizing muscles to combat the inertia of any movement. The fact that they don't leave footprints I think is probably just because vampires are so beautiful that the dirt feels unworthy to touch them so they actually don't come into contact with it. We don't need to do a meta-analysis of this data to conclude from all of the barely-contradicting points you bring up that vampires are very heavy.
Vampires are all different, of course, and we have a limited sample. I think this means our best option for answering your question meaningfully is to do a case study. The best vampire is Edward Cullen and we know the most about him so we will use him as our subject. We know he is 6'2'' and that he is fairly lean (we know he has a "sculpted, incandescent chest"). For the sake of our study, we will estimate that he has a body fat(-like substance) percentage of about 10%.
From this information, we can estimate his body volume. We don't have an Edward to put in a bod pod, but we do have a sample bod pod output for a man who is conveniently about 6'2'' whose total volume is 90 litres. This man has a body fat percentage of about 15%, so we will want to subtract one third of his body fat, or about 11 pounds worth to match Edward's lean physique. Because body fat is about 2.2 pounds per litre, that would make his total volume 85 litres. We will also want to subtract the thoracic volume because it's air and not body tissue, about 4.5 litres. So our estimate is that Edward's body volume is 80.5 litres.
Clearly, vampires are made out of different substances than humans. We know that Edward's skin is literally scintillating, so it probably is made out of literal diamonds. The integument is about 15% of the total body weight and contains both lean and fatty tissue, so let's assume it also comprises about 15% of total body volume. In the case of Edward, 15% of his body tissue volume is about 12 litres. The density of diamonds is about 3.51g/mL.
3.51g/mL*12,000mL=42,120 grams, or 42.12 kilograms, or 92.9 pounds.
Next, we can probably assume that because Edward is literally ice cold to the touch, his blood is made out of molten gold. Stephenie Meyer's vampires are paradoxical like that. Also, this probably explains the etymology of the phrase "heart of gold". The average man has about 5 litres of blood, and the density of molten gold is about 17.31g/mL.
17.31g/mL*5,000mL=86,550 grams, or 86.550 kilograms, or 190.1 pounds.
We will assume that the rest of him is made out of marble. The density of marble is about 2.7g/mL, and we have 63.5 litres left of non-skin, non-circulatory substance.
2.7g/mL*63,500mL=171,450 grams, or 171.450 kilograms, or 378 pounds.
If we tally all of those weights up, we get a total of 661 pounds. That's right folks, Edward Cullen weighs a grand total of 661 pounds. That would put his BMI at 84.9.
Conclusion: Edward has super obesity.