That's pretty good. The Model S has always had a bit low carrying capacity.
Checking a random Model S D here, it has curb weight of 2187 kg/4821 lb and a max allowable weight of 2670 kg/5886 lb, which means max 483 kg/1064 lb of cargo. If you load up five adults weighing 75 kg/165 lb, that means that the total amount of luggage can't exceed 108 kg/238 lb, or 21.6 kg/48 lb per adult. You can easily exceed that with a ski box, five pairs of skis and a small overnight bag for each person.
The Model X should weigh somewhere around 2500 kg/5500 lb, so that should mean that you'll be able to carry around 778 kg/1700 lb. Counting seven adults at 75 kg/165 lb means you'll be able to carry 253 kg/557 lb of luggage, or 36 kg/80 lb per person. That's much better.