Without applying some formula or other you can only really compare fill weights if the down is the same fill power (using the same scale, either standard or US) and quality. So if both bags are the same size and have 800fp 95/5 quality down then that's fine. If one has 700fp and one 800fp then things become a little tricky.
There's two "standards" for fill power, US and EN, with the US figure being roughly 50 higher than the EN value for the same down sample so US 850fp = EN 800fp (roughly). This means that a bag/quilt with 450g of 800fp (US) won't be as warm as a bag/quilt with 450g of 800fp (EN) all other factors being equal.
The bag also needs the cells holding the down to be big enough for the down to fully loft but not too big that it can move around. If you look at the details on Cumulus' site for their quilts they give both weight of down and of the shell - the latter increases ever so slightly, by about 20g, as more down is added to allow for the increased loft.
If Cumulus have calculated that an extra 70g of down is required for the long/wide version to have the same rating then I'd be inclined to believe them.