Banjos are known to be quite heavy.There used to be a common saying in the banjo world that "a good banjo is a heavy banjo". They can make a Les Paul guitar look light. This is often the case, but not always. There are a number of types of banjos and all of them are not heavy. But classic bluegrass style banjos are heavy.
Most people think that what is causing the banjo to be so heavy is the resonator (the back of the banjo). This is not the case. The majority of the weight on a heavy banjo is from the metal hardware. Resonator banjos are usually heavier because they usually have a metal flange to attch themselves to.
The biggest weight producer is from a banjo with a cast bronze tone ring. Quality bluegrass style banjos usually have this feature. This gives the banjo a rich, bright tone that is wanted by most bluegrass banjoists. This is what was in the prewar Gibson Mastertone banjos (which are long thought to be the Holy Grail of banjos). These type of banjos can weigh in between 11 and 12 lbs.
Recently there have been some bluegrass style banjos that are not as heavy. One of these is the Deering John Hartford banjo. This banjo features a wooden grenadillo tone ring. This cuts about 2.5 lbs off of the weight. Another new model that has a great bluegrass tone but not the weight is the Deering White Lotus banjo. This banjo is made of all white oak wood. White oak is a very bright tonal wood so there is no need for a bronze tone ring.
Clawhammer banjos on the other hand are generally not as heavy. They are most often an open back banjo so there is no need for a metal flange. Also, clawhammer/old time players generally like a darker/plunkier tone than bluegrassers. The banjos these players use often don't have a tone ring at all, or they use a smaller tone ring such as a brass tube.
When looking for a new banjo, I would suggest not going just for a "heavy" banjo. I suggest going with the one that fits the sound you're going for best.