Hello Bernard,
When I remember to check the capacity of a decanter, whilst washing it, I use a wine bottle and have never thought to do the obvious and go on to check the capacity of a standard wine bottle in fluid ounces - which is 75cl. in decimal capacity.
As a result I've just gone and looked at my trusty Pyrex measuring jug and estimate that 75cl equals 26 fluid ounces. I therefore suggest that your decanter is not full size.
By-the-by I checked the index and appendices of McConnell for 'capacity' and 'size' to find no reference. I haven't read the whole book, but from what I have I do not recall any mention of capacity.
Hope this helps, Nigel