Both my cats were HUGE while growing up as well. They are currently 2-1/2 years old and each weight a little over 14 pounds.
Our vet told us that they very possibly are part Maine Coon because of their size. At the age of 6 months they were as big as some adult cats I have seen!
I have to be running off for some errands in a bit but when I get back I will post some pics just so you can see what I mean.