How is water heavier than petrol, even though its molecular weight is less than petrol? Molecular weight of petrol is so much higher than water,
but when it comes to physical property, weight, one litre of water weighs more than one litre of petrol.
How is it possible?
 A: Because water molecules are small and pack tightly together, causing water to have a greater density than petrol.
A: Density relates to the mass per unit volume. If your molecules are heavier but take up more space, the net result could be more or less mass per unit volume.
When you look at a typical hydrocarbon, it has a lot of carbon and hydrogen.  Now atom for atom, oxygen is heavier than carbon (ignoring isotopic abundance, roughly a 16:12 ratio). So if the molecules were otherwise the same shape, if we replaced the carbons with oxygens the hydrocarbon would become heavier (you can’t do that of course - the chemistry is different).
But the larger and more complex shape of the hydrocarbon molecules has another effect. Imagine two elevators. In one elevator we cram a squad of ballet dancers - tall, elegant, and able to be packed very closely. In the other elevator there are a number of people who just went shopping - they carry big bags and generally take a lot of space. It’s quite possible you could get 15 ballet dancers into the first elevator and only five shoppers in the second. So though the dancers might weigh 100 pounds each and the shoppers 200 pounds, the first elevator car will be heavier.
The same analogy can help explain why density of most materials goes down when temperature goes up. Imagine the dancers hear music and start to dance. Suddenly 15 of them aren’t going to fit in that elevator! 
Maybe that’s why elevator music is usually so awful?
A: One of my Chemistry teachers had an interesting way to describe this type of phenomenon.
Because the water molecule is shaped like a triangle, it is possible for the water molecules to "snuggle up" with each other, and pack themselves close together. Like this:
 >>>>>>>>> 
He claimed that Water molecules behave more like H(100) O(50) that H(2) O.
Now, I'm sure we could think of a dozen reasons why this isn't correct, but: this does show a unique property of water. It relates closely to the answers provided by @Floris and @G. Smith.

If water really were H(100) O(50), would that explain the property of your original question? I believe it would.
$0.02
A: Another approach to answer this for youself might be to run with the idea and look at molecules with even higher molecular weight. 
Like, why aren't plastics far heavier than lead? Why isn't DNA? These are truly massive molecules, yet their densities are pretty low (comparable to water). 
With polymers, you should pretty quickly get the idea, that the arrangement of the atoms does matter. While the molecules themselves are very heavy, they also take up more space and hence are less dense. With petrol, the usual molecules are not that much bigger than water, so arrangement effects are not that obvious. 
On top of that, intermolecular force also play a role - here water's attraction to other water molecules comes into play. Plastics on the other hand do leave holes and often are not particularly strongly attracted to each other, leading to lower densities. 
A: Picture this. You have a basket of foam balls and a basket of wooden balls. Say each basket has the same volume and each ball the same volume as well. Stuff as many foam balls as you can into the 1st basket and do the same for the other basket but with wooden balls. Each of the foam balls weigh 10 grams, and each of the wooden balls weigh 20 grams. You find that you can stuff 3 times more foam balls into the basket than wooden balls. Doing the math, you find that:
10 grams * 3 times more balls > 20 grams.
30 > 20.
This means that even though the foam balls weigh less than the wooden balls, they can be packed together more densely, resulting in a larger total mass than the wooden balls. Hope this clarified anything!
A: Water is denser than petrol, so there essentially more molecules of water in a given volume.
For example in a given volume, there may be 15,000 molecules of petrol but 20,000 molecules of water.
