Japanese futon are different from the normal futon sold in the UK. They are made entirely from cotton and so they have different characteristics to other futons available.
When the futons are taken from the box they should feel very fluffy. The depth of a new futon can be up to 15cm
As the futon is used the cotton fibres will be compressed and the futon will start to firm up and the support given to the body will also increase. This is considered by the Japanese to be the best state for a comfortable and yet well supported sleep.
When the fibres are more settled over a period of weeks the length and width of the futon increases slightly.
The sizes given for the futon are the sizes of the empty covers before the cotton filling is put inside.
When the futon is first used the actual sleeping surface area can be up to 10cm less than the cover size as the puffed cotton core reduces the length and width. With time and use the dimensions of the futon will increase although it should be noted that the overall dimensions will be less than actual cover sizes.