Why, yes, that would be The Amazing Colossal Man, which somehow managed mysteriously to escape our memories and those of everyone who voted in last year's worst Vegas-themed movie of all time Reader Poll. We can take solace in the fact that, evidently, we're all far too young to have seen it.
With the fabulous tagline, "Not even the U.S. Army can keep her away from the man she loved--but only the U.S. Army can save her from the monster he became!", The Amazing Colossal Man was released in 1957 and starred Glenn Langan and Cathy Downs. From the synopses posted at imdb.com, it sounds like a real classic:
"Lt. Col. Glenn Manning is inadvertently exposed to a plutonium bomb blast at Camp Desert Rock. Though burned over 90% of his body, he survives, and begins to grow in size. As he grows, his heart and circulatory system fail to keep pace with his growth, and he is gradually losing his mind as a result of reduced blood supply to his brain. He reaches 50 feet tall before his growth is stopped. By this time he has become insane. He escapes and wreaks havoc upon Las Vegas before he is finally stopped."
Our kinda movie!
We read another online review that describes how the 60-foot-tall, 18,000-pound giant peeks in on a woman taking a bath on the top floor of her hotel and tears off the icons on top of the Silver Slipper casino. When his would-be captors ram him with a humungous hypodermic, he apparently pulls it out and throws it javelin-style into the heart of one of his doctors, killing him instantly. He then picks up his fiancée King-Kong style and heads off for the Hoover Dam, where he's finally taken out by Army sharpshooters and falls into in the Colorado River.
The whole movie sounds like a typical product of Cold War paranoia, without conveying any particular message, aside from, "Don't mess with the big radioactive guy." Apparently, no explanation is ever given for his crazed attack on Las Vegas. Most bizarre of all, this schlocky sci-fi actually spawned a sequel the following year, titled War of the Colossal Beast.
Lastly, here's a little Colossal Man trivia for you, courtesy of Wikipedia:
So, there you have it.