Am I being a bit thick in assuming that 1.6m3 is simply double 0.8m3?
No, but you have to remember that some stupid people use a different measure for volume ( and indeed area) sometimes..
They state a single dimension and then say “cubed” after it. Meaning a cube of dimensions x,x,x rather than a volume of xm^3.
In my language, a cube of dimension 0.928m is 0.8m^3 m
But a stupid person may refer to a cube of 0.8m as being 0.8m cubed, even though it is actually only about 0.5m^3
Thus your woody may be comparing a lump 0.8 x 0.8 x 0.8 = 0.512m^3 with one that measures 1.6 x 1.6 x 1.6m. Which is of course 8 times as big. And thus a bargain 🙂