I wouldn't think weed is a significant part of Silk Road.
Weed is really easy to get for any person that does drugs. And it's not as easy to mail either .
I don't know if there are any stats available but would imagine weed is one of the least popular drugs to buy on the market. At least for Americans.
Rarer, hard to come by drugs are probably more popular on Silk Road in rates disproportionate to their general use. Stuff like rarer psychedelically, DMT, high-quality MDMA and all that other groovy stuff.
The road was a god send for me and my weed. As I got older my "hook ups" disappeared, or grew up themselves, making it more difficult to get good product as I become more domesticated as it were. ]
Now I don't have to harrass a friend, give them money, wait etc... I just order up what I want. And being domesticated and all, have an income where I can get larger stashes that last a good while between orders.
I haven't really poked around the other areas too much on the road but the mary jane section has TONS of items and sellers. I would actually guess the opposite, that weed is a pretty big seller on the road.