We moved this year into a 3BR with a big bonus room and office (no door, no window, no closet, so it's not considered a "bedroom") with a huge unfinished basement/garage (room for three cars, plus it's roughed-in for a bedroom and an additional bathroom.) Kids are 6 and 3, with a baby on the way and plans to adopt at least two more in the coming years.
The kids had previously shared a room, but now that they have their own rooms, still prefer to share a room. (Actually, my older one would probably prefer to sleep alone, but her little brother feels the need to sleep in the same room as her.) They very rarely spend any time in their rooms - the bonus room is mostly theirs (with room for some of my craft stuff), we do our homeschooling at the dining room table, and one day we will get around to finishing off the basement to give them some more room, especially when bedroom space starts to get tight.
As far as having more space and filling it up with more stuff, we actually have gotten RID of a lot of stuff. We like that the more we get rid of, the more floor space we have. I realize that that's probably abnormal, but it has worked for us. My MIL lives alone, with about 1000 square feet LESS than we do, and every inch of it is covered in furniture and "stuff." She is constantly on us to get another end table or "something" for the foyer, or to decorate our "plant shelf" above the cabinets, but she sees empty space as space to put more stuff in. Her house literally has a piece of furniture lining 90% of the wall space - curio cabinets, tables that hold picture frames, random desks and shelving and whatnot. Some people just need to live like that; we are not like that.
And as far as payments and whatnot, the kids and I spend a LOT of time in this house. I am totally a homebody. I generally only leave to go grocery shopping in the winter, and we spend a lot of time on our own property (outside - our house is on 4 acres) in the summer. On weekends, when DH is off work, we'll do day trips, but it's really important to me that I have a comfortable home, where we all feel, I dunno, homey. I love to travel, but my home is my favorite place to be, and DH and I were both okay with spending a little bit more money to ensure that we got a place that we love.