The heavy parts are indeed the batteries.
It is not so much a different material as it is a different type of voltage/Amperage output. A laptop battery only puts out DC voltage and it is only a specified voltage/amperage that the laptop needs to run.
On a UPS, the batteries have to convert the DC voltage to AC voltage at 120V (US) therefore requiring more AMPs to convert. Think of the UPS as a temporary plug in at your house.
When you plug in your laptop at your house, you are converting AC to a lower DC which takes less energy. When you use your UPS, you take DC and raise it to AC which uses far more energy.
Amperage and Wattage is the key. The more amps, the faster the batteries discharge which is why UPS's don't last a long time without power.
As a side note, your laptop charger (AC) uses much more power that your laptop battery (DC).