Wow, Siduri. If that's true garlic I need to know the name of the variety, being as it's about twice the size of what I grow or buy locally.
Here in the States, garlic 2-2 1/4" in diameter is classed as "large," and that's the size I strive for.
Another possibility is that, at this time of year, you are buying fresh (i.e., uncured) garlic, and it will weigh more because of the moisture content.