That is a good point to always remember, the heavier a NiMH battery is the more capacity it has.
Those numbers on rechargeable batteries as you may well have imaging are a measure of their capacity which is rated in Milliamp Hours (mAh).
I recently purchased some NiMH batteries off a seller who has nearly 100% good feedback, it seems that 99% of people who are being sent batteries below the capacity they were advertised at, didn't realize they have been conned.
I was first suspicious when I open the packet and the pack batteries felt really light, so I weighed them, Four Energizer rechargeable 2100 mAh Batteries weighed about 2oz (50g) more than these "2600 mAh" batteries. To test the batteries I took 2 completely discharged batteries and raced them in my Fast charger, for this I used 2  of the AAA batteries I had purchased, again a heavy one from a reputable source against the one I had just bought off an eBay seller, which felt really light.
The heavy battery took 43 minutes, the light one took only 12 minutes to charge. This was exactly the result I had expected and told me that I was right to make the assumption I did when I felt it was so light. My 100mAh battery was probably in reality about 300-350mAh or it may have in fact been a NiCD battery, I don't know. But what I do know is that I had been sent fake junk.


The Great C and D Cell Battery Rip-off
Another thing to remember is that the higher the mAh ratting the more capacity a battery has (unless of course its a fake like I was sold) C and D batteries are bigger than AA batteries so that they can carry more capacity, that is only reason why they are bigger, so they can carry more power for power thirsty applications, there is no other reason why they are bigger so if they are the same capacity as much smaller batteries the additional size is pretty redundant..
However C and D rechargeable batteries are often sold at the same capacities as AA batteries e.g. current Uniross D cells are 2,600 mAh, this is pitiful amount for a battery of its size and less capacity that their best AA batteries. and a complete rip off, a good D cell should have at least 9,000mAh (4,500mAh for C cells), however most mainstream battery companies will not sell C and D cells at the capacity they should have, this means that most C and D rechargeable batteries are often quite useless, and I would probably guess its one of the reasons why rechargeable batteries are still thought to be inferior by many people.

This image shows an Energizer rechargeable D cell battery that has been opened up revealing an AA battery inside.

There are however many sellers on eBay who sell 5,000 mAh + C and 10,000 + mAh D Batteries. I bought a pair of 4500 mAh batteries a few years ago, and the battery life was a revelation compared to the energizers I had been using, you will need an automatic (rather than timed) charger to get the best out of them.