Most people wear a helmet that is at least 1 size too big for them. Most people just do not understand how a helmet is supposed to fit/feel. This creates a lot of issues, one being more noise. Others being the helmet moving around on your head when buffetted and coming off the rider in an accident.
The rule of thumb is, the more bobbles (vents, etc.) on the exterior of the helmet, the more noise. But there are other factors involved.
Even with the least noisy helmet available the sound levels on a motorcycle are enough to damage your hearing. That is why ear plugs are always recommended.
Here is a good article by WebBikeWorld. This group also tests and evaluates a lot of helmets and they always comment on realtive noise levels.
http://www.webbikeworld.com/motorcycle-helmets/helmet-noise.htm