If they're about warmth, then maybe keeping the air volume to a minimum is part of the function, so maybe curling up was a part of it. The other part was probably that people were statistically shorter in the middle ages.
The one pencil sketch gives you some idea. I think they just have a smallish opening — it looks like there is room to the left and to the right (inside) of the shuttered area.
People were smaller on average. But also, I've read it wasn't uncommon to sleep with legs/knees pulled up somewhat (maybe using pillows?). And that the changed bloodflow due to non-horizontal sleeping position (legs above head) contributed to higher prevalence of brain strokes.
Don't know if the latter is true. But doesn't seem too implausible - life back then was generally less healthy & more dangerous.
We clearly haven't been to the same castles. Also, you could always curl up in a bed or sleep in a sitting position (which or may not have been common in Western Europe a few years ago). If you go visit museums that have full suits of armour that were actually worn by knights and kings, you'll be surprised. You clearly had plenty of tall people. Average size was certainly much lower than today, but spread was higher and you had plenty of tall, well-fed nobles. Charlemagne was apparently 1.84 m, Francis I of France was almost 2 m tall, Sancho VII of Navarre was even taller than that.
incomplete argument. those could have been the beds of their children. not saying you are wrong, but you would need to substantiate it better, imo, to convince.