I hear ya about that!!! I'm 6ft tall, which is tall for a lady, and I'm also somewhat er... fluffy. My hubby is 6'2" and my boyfriend is almost 6'6". His wife is average height but also of the somewhat fluffy variety, as is my husband's girlfriend (do we see a pattern here? lol) So sleeping together at least on a queen sized bed isn't always so comfy! We do manage it occasionally, however. Great news is, boyfriend and wife got a NEW KING SIZED bed today. I'm so excited! No more being squooshed into the wall!
That being said, we don't all sleep together, certainly not all the time. I will generally sleep on the couch when my husband's girlfriend comes over a) the bed isn't really big enough, and b) I like to give them the opportunity for alone time because they don't get it very often. She has kids and another SO, so when my husband goes there to visit they don't get much time to themselves.
When I go visit my boyfriend and his wife, we sleep together about half the time, and occasionally one of the two of them will sleep on the couch if they want to sprawl a little.