Hi Olga - good question.
Cramped can refer to any space that is too small for its needs. For example, "The boat is cramped" - the boat could have 3,000 people on it, so it is not small, but it is only supposed to have 2,000 people, so there is not enough space for everyone. In a room, "cramped" could be temporary (just when all of the family are inside) or permanent (so small, that it does not serve the function as a room and ALWAYS feels cramped).
"POKEY" on the other hand is much more descriptive, to me. It is only a permanent adjective to describe a small, often dark and definitely undesirable room. A pokey room will be cramped, but it tells me more than just about the space - it maybe has a low ceiling, no window, etc too.
I hope that helps.
Tom