IF one assumes rotor blades to be of same size (which they may not be) then the fuselage is quite a bit bigger. Not only over 20% wider, but also some 6-7% longer. Since the cabin seems to begin at the same point from the tail, if one excludes the tail, the ramp door to tip of nose length is 15% longer. (that doesn't include the radar protrusion on Z8!)
For comparison, Mi-38 has 4200 kw of power, achieving 16.2 ton MTOW. AW101 has 5650 kw of power, allegedly achieving only 15.6 ton MTOW. It's likely not an apples to apples comparison as figures come from two different producers, perhaps with different methodology, and other variables such as rotor blade design and gearbox effectiveness also influence the MTOW figure.
AC313 has 4340 kw of power, for its 13.8 t MTOW. Makers said the type could achieve 15 t MTOW, but without context. Whether that's to be achieved with more powerful engine or through other means. If this new variant has, say, WZ10 engines, with their alleged 4800 kw power (3 of them) then MTOW might be anywhere from 13.2 tons (AW101 ratio) to 15.2 tons (AC313 ratio) to 18.5 tons.
While AW101 power to mtow ratio is peculiarly low, it may stem from different valuation of the term MTOW and measurements in diffrent conditions. (hot weather?)
While mi38's power to mtow ratio is incredibly high, it also may stem from the same issues, plus a 2 engines converting power to a single shaft may be more effective way of transfering all avilable power than having 3 engines connected to the gearbox and converting it to a single shaft. (single engine helis can have ratio that would, if used on 4800 kw power, yield a MTOW of 21 tons. theoretically speaking.)
The sponsons on the new helo (i'm not sure one should refer to it as z-18 family anymore, as not only is the hull new, the control surfaces larger, but likely the engines and powertrain, perhaps even rotor blades have been changed) are really massive. I see little use in having them empty. And landing gear can use only so much space.
Mi-38, without a meaningful sponson, holds 3 tons of fuel. AW101 holds little over 4 tons. It's unlikely sponsons on this new helo are the only fuel tanks. At least a ton or two are likely in some central tanks within the body itself. It's always better to have fuel as close to center of gravity, if possible. If the extra fuel was only for ferry missions, self deployment missions, having external fuel tanks that can be detached would be the better way to go. Thus the only logical explanation to me is that sponsons are meant to hold regular, internal fuel. Probably not all the time, for all missions, but certainly for some missions where range is preferred over payload. certainly, one can't expect the helo to use both maximum fuel and maximum payload. 6-7 tons of payload should not be out of the question. Theoretically even over 7 tons, if the engines are really something new and powerful. With likely 10 tons of fuel, and possible 8-10 ton empty weight, 15-17 ton mtow might be expected. Theoretically going to 18 tons, again IF engines are something new and powerful. Overall design and size of the helo certainly allows for such performance. I'm not that sure that PLA really needs a very roomy helicopter with A LOT of range that doesn't really lift any more payload than current Z18.
