I think the operating word here is not "mattress" but "kitchen". How cramped do things have to be to need to put the guest mattress in the kitchen?

In most of the Eastern block the accommodation given to a family was sized for that family. Few had the luxury of a completely unused room.

The kitchen was routinely used as a room for two reasons, one that it was obviously a room, the second because it was easy to heat with the stove being right there. A lot of families were using the kitchen as permanent living space, usually relegating the grandparents to that worst room so the young ones could get a decent start in life.

They were 19 and 25.

It doesn't seem that crazy that there would be very little space. Visiting parents and/or grandparents probably got the bedroom, some friends the living room.