I'm actually not sure. When SO and I first moved in together my fixed expenses stayed about the same, but expenses like eating/drinking out went way down because I moved to the NJ suburbs to live with him. Even though we now live in a big city we still cook at home every night but friday, etc, so that's still good and most likely a net positive for my finances.
SO is definitely much more of a spender than I am. He had a fair amount of debt 10 years ago when we met, but he paid off the last of that a little over a year ago and has also been building a decent 401k all along. However, I have a significantly higher net worth and unless he really ramps up his savings I probably always will. Almost all of our furniture is stuff he purchased because he wanted to have nice stuff. My den still has my old futon and bookcases and such that I bought before we met. He desperately wants me to get a better computer armoire but I am fine with the used one I bought from a friend for $40 almost 20 years ago.
We keep separate finances and probably always will due to our different spending habits. He makes roughly 50% more then I do so I don't think it's my place to tell him to be more frugal. He lives within his means and is saving for the future. Just not as fast as I would if I had his income. I don't spend any more now then I would if I were single so, on paper at least, I shouldn't feel less secure, but somehow I think I do feel less secure. Maybe it's concern that if he became disabled or long-term unemployed for some other reason that would leave me responsible for more expenses than I am now. After all, if I had to support us both it's not as though we could move into a 250 sq foot apartment like I lived in when I was single. (well, we could, but I'm sure you'd be reading about the homicide investigation within a few months of the move so that would probably be a bad idea...)