I don't think it's about this to be honest. My collection is constantly growing but when I can I usually buy the smallest bottle available down to about 20mls, at least when it comes to modern scents. The only time I buy big bottles is when I'm replacing something I love and wear frequently, buying backups of vintage/discontinued scents I want or shopping at a discounter who only offers larger sizes albiet at prices equal to or less than the cost of a smaller bottle elsewhere.
Definitely a lot of interesting answers. I suppose it makes more sense to those of us who can afford to keep upsizing. I guess when I asked this, I had in mind the rising cost of tuition, moving out, etc.