I calculated the Surface to Volume Ratio of a 12 inch cube as follows.

Volume 12x12x12=1728

Surface Area 12x12x6=864

Ratio 864/1728=1:2

I then calculated for the same cube but measured in centimetres (30)

Volume 30x30x30=27000

Surface Area 30x30x6=5400

Ratio 5400/27000=1:5

Could somebody please tell me why I get a different value for the ratio?

What am I doing wrong?