I'm a 46 yr old 240lb guy who rides a Trek gravel bike. Its beefy and has 700 X 45 tires.
I'm looking for a vintage steel bike because of their reputation for being smooth rides. I found a 1990s Bianchi Eros that has a 25crmo4 frame and Campagnolo mirage component group. This thing feels like its gonna fall apart if put my big butt on the saddle. The bike is in great condition, but I'm used to big thick frames and beefy wheels.

Is it all in my head, or what? Any opinions are very welcome.

Providing the bike hasn't been abused, it's all in your head. I'm 64 yo, 245#, 6'6" in height and ride an '84 Peugeot that I bought new. It is steel-framed (Carbolite) and I have no worries about it.
Jon

Good to hear. It's like going from riding a horse to a greyhound. I'm really considering buying the Bianchi Eros. I appreciate the advice.
Todd

My weight has peaked at 245 or 250, I have been using steel road bikes my entire life and never had a problem. I wouldn't worry about it.

Skinny tubes, are cool. Here is my 1986 KHS Tange 900 tubeset. Upgraded heavily over the years, now with a 2x10 drivetrain, 700 conversion rolling 700x25 tires... A good quality steel bike will do fine...

I do have other bikes, but that old KHS has supported me through thin and thick, and now thinning... And by other bikes, my aluminum 29er on 54s, is not fazed by my weight, even when I was over 300 pounds. I have a couple aluminum road bikes that have supported me as well. I crashed on one a few years ago, on 700x23s, I won't ride 23s now, jsut like I'm having fears riding downhill without the brakes locked down tight...

in your head....... I have been up to 285 and no problems with my miyata 1400 (89) Now riding a 90/91 derosa at 260