Most of that has to be the water. I didn't think it added that much though. 65gal is only a little over 10cf which is like 150lbs dry. I took it from the car to back yard in 2 trips on my shoulders so it couldn't have been a huge amount
One thing to note is 100% of that soil is not going to be moist for long. If it were moving around even 10gal pots would be a lot harder than it actually is! Yet its probably closer to 20-30lbs versus 170lbs by your calculation.