[CarolineW]

    ... the first stage of Falcon 9 v1.1 is
    capable of holding 119,100 Kilograms of
    Rocket Propellant 1 and 276,600 kg of
    Liquid Oxygen.

(From http://spaceflight101.com/spacerockets/falcon-9-v1-1-f9r/ )

So even if it's 99.9% empty, it still can have 120 kgs of rocket fuel. That'll go with a bang.

And it's unlikely to land with only 0.1% of its fuel left - that margin is too small to be attempting a soft landing, so I'd expect it to be a lot more. Then compare with just 5 gallons (probably US gallons, about 19 liters, close to 17 or 19 kgs) of ordinary gasoline:

* https://www.youtube.com/watch?v=C72bv2YhZ6A

* https://news.ycombinator.com/item?id=10924521