The calculation of 25 people uses a twin mattress as a comfortable fit for one person, not two.

In the US, a twin mattress is the smallest adult size available, and is almost always seen as a one-person bed. I usually see queen-sized mattresses marketed as the average-size bed for two people, and full (one size up from twin) recommended as the comfortable choice for single sleepers.

http://bettersleep.org/mattresses-and-more/mattress-sizes has a good overview of mattress size standards (at least for the US).