Kansas City, Mo. ? Clutching the $80 ticket you bought months ago, you plow through the sold-out crowd to find your seat at the Sprint Center and drop into it.

Yikes! It’s a tight squeeze for your derriere.

Maybe it’s all those oversized Cokes and fries you’ve been eating. Or maybe the problem could be the seat.

One-fourth of the Sprint Center’s seats are narrower than seats installed in many other arenas built in the past decade.

Just like Americans, seats have been getting larger in new stadiums, arenas, movie theaters and even cars. After all, middle-age spread is no longer confined to middle age.

But limited space for the downtown arena forced designers to make about 25 percent of the seats, especially in the corners, only 18 inches wide, Sprint Center spokeswoman Shani Tate Ross said. The rest in the bowl are 19 and 21 inches wide.

But the Sprint Center had to be built on only 8 1/2 acres, and that put space constraints on designers.

Kevin Reichard, publisher of Arena Digest, a trade journal, said the 18-inch seats at the Sprint Center were smaller than the 19- to 21-inch seats normally found in newer arenas.

“As a new arena, it is surprising. But they need to cram a lot of seats in a small footprint,” Reichard said.

John Striplin, of Independence, Mo., definitely noticed the smaller seats when he attended a Kansas City Brigade game this spring. He said he found them too narrow to sit in.

“I had to stand for the whole game,” he said.

You have to pay a premium for the suites’ 23-inch seats.

That’s now typical, Reichard said.

“Club-level seats are 22 or 23 inches,” he said. “Americans’ butts are bigger than they were a generation ago.”

A federal government project in 2000 found the average American woman had a hip width of 19.7 inches, which was 2.6 inches greater than in 1962. The average American man in 2000 had a hip width of 17.2 inches, which was 1.3 inches bigger than in 1962.

Although the project hasn’t been updated, a government official said last week that the average American weight has increased more than 10 pounds since then, so the average hip width is greater now.