Wow, this is a real personality issue. My first marriage I had solitare that wasn't really large, .33 of a carot but I work with my hands a lot, knocked the stone out (in my sewing room of all places), had to have a more substanual setting and it was kind of a hassle.
I'd always say quality over size (but then I worked for a jeweler at one time) the beauty of a good quality stone really shows taste. But some people are going for flash - they want something that will catch people's eye. Men and women. Some men really want their lady to have the so called "rock" as a sign of their affluence, I guess. Or to ward off competition?
The expense is another judgement call, working in financial services I saw couples who years later were still paying off the wedding, including rings. It just seems silly to start of with a load of debt that could undermine the marriage. In many case what they spent on all that would have made a good down payment on a house.
I assume you know all of this and are looking for a more emotional side of it?