Since I came to U.S 8 years ago,We'd go to Culver's at least twice a month religiously but I noticed since two years ago, the quality is not there anymore. When I ordered two piece of chicken (white) ,the breast would have a lot of meat that would last me two days since I ate them with rice. The burger would be big and looked 'buff up' if you know what I mean. Last week we went there and it was such a disappointment .Not only my chicken breast barely had any meat on it (You would think the chicken got hit by recession and had to starve itself too if you saw its condition!), my husband burger was worse!. the meat was so thin and the burger just looked flat. It just looked sad!
We don't have Culver's over here in England but your experience is very similar to the burger sellers over here, it is very hit and miss. Finding a quality burger with a quality bun is not easy and you usually have to pay a little extra if you want a 'proper' burger with 100% flamegrilled beef and a nice brioche bun.
Some of the fast food burgers here in the US are paper thin. The basic McDonald's hamburgers have hardly any meat, but at least if you spend more ,their larger burgers are more substantial.
I am disenchanted with Culvers, as well. I used to love their fish sandwich but it has gotten so expensive! We both switched to ham and cheese and while it is delicious, it is greasy as heck! It's the prices that gets to me the most and their burgers are slim, as you said. My husband still like the butter burgers but I don't eat there if I can help it. It certainly is clean, though, and they are quick--too bad the food is not great.
We like Culver's hamburgers. They taste delicious. However, we do not go there very often due to the higher priced items. We eat there maybe once every six months or once a year. Due to this limited amount of visits, I have not noticed a lower quality of the beef patties nor the chicken. I have noticed that they automatically give you a double burger or double patty even though you don't request it. I always have to make sure I say single patty. If you don't specify single patty, they charge and give you the double patty that costs quite a bit more money.