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Wikipedia states:
The telescope has an expected mass about half of Hubble Space Telescope's, but its primary mirror (a 6.5 meter diameter gold-coated beryllium reflector) will have a collecting area about five times as large (25 m2 vs. 4.5 m2).
Hubble telescope is about 11.000 kg, while J.W. - 6,500 kg. It's also larger. Hubble: 13.2 × 4.2 m vs J.W. - 20.1 m × 7.21 m.
Why such a big difference? What was so heavy in Hubble that is so light in James Webb?
Here is a weight breakdown of the HST:
For JWST, the telescope plus instruments weigh 8686 lbs (3940 kg), leaving 2500 kg for the spacecraft.
As far as the primary mirrors are concerned, HST's mirror was made out of silica glass, and had a mass of 826 kg.
The primary mirror of the Hubble telescope measures 2.4 m (8 ft) in diameter and weighs about 826 kg (1820 lbs). It is constructed of ultra-low expansion silica glass and coated with a thin layer of pure aluminum to reflect visible light. A thinner layer of magnesium fluoride is layered over the aluminum to prevent oxidation and to reflect ultraviolet light.
Each mirror segment for JWST is made out of beryllium, and has been made as lightweight as possible by cutting away most of the material from the backside of the mirror.
Beryllium is lighter and stronger than glass, so this honeycomb structure can be very thin. Each mirror segment is just 20 kg, not counting the actuators and backplane. For 18, that's just 360 kg. Counting the actuators and backplane, it's probably roughly that of the HST primary mirror.
A beryllium mirror segment is 20 kilograms in mass. (A full primary mirror segment assembly including its actuator is about 40 kg.)
