Actually Fido, the drum has the same capacity as the standard Hotpoint drum and it's bigger than the washer dryer drums, the 1400 models simply needed strengthening around the front of the drum so hence the need for a thicker rim that overlaps the tub, so I am told. Hotpoint apparently had problems with using the standard thin rim drum in 1400 machines.
Also Hotpoint started using the washer dryer cabinet on the 1400 models to facilitate the building in of the full panel door models so that the inlet pipes could be fitted from the side. It saved them money on designing a new body shell for just that model.