Oops I always forget to put in the part I'm talking about. Here is the sizes, serving amts and then another chart I found which seems more reasonable. What do you think?
Round
6 = 12 - 8
8 = 24 - 15
10 = 38 - 30
12 = 56 - 45
14 = 78 - 65
16 = 100 - 90
Square
6 = 18 - 12
8 = 32 - 24
10 = 50 - 40
12 = 72 - 60
14 = 98 - 84
16 = 128 - 112
I always use the indsutry standard of 1x2x4. If they want a larger serving then they can order more cake.
When you're looking at a 6" you have to look at cutting squares out of it not wedges. You can get the wedges, but it's hard to get 12 because the are so thin once you do cut them and it's hard to handle (trust me i've tried)
here is wilton's chart for the 1x2x4 serving
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm
If you're looking at a 6" cake and envisioning cutting 12 pie-shaped wedges out of it, then I understand your confusion.
But it it's cut properly, in the 1x2x4" pieces then you can easily get 12 pieces out of it.
Click on my "how to cut a wedding cake" link in my signature to see how.
I know its squares your cutting, it just always seemed small, but that link really helped me visualize the piece, thanks!
I think it's because when people hear "one inch", they think "paper thin". If you think of it as about the size of a folded over peanut butter sandwich, it's a better visual. When I'd explain it that way to clients, most would do the "OHHHHHHH! That's ok, then!" thing. 
In my head I have a hard time with size concepts, my brain isn't wired that way. When you look at a 2in tall 6in pan you think that cake will serve how many? But you have to remember how tall the finish product is.
I have seen people on here that have taken styrofoam or wood and had cut into the sizes to give people a better visual.
You can also bake up a 6inx4in tall cake and cut it up to see how big the pieces actually are.
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