Your butt's not getting bigger. The jeans are getting smaller.

Just ask shopper Lisa Korn, who is doing a torturous tango with a pair of skinny new jeans inside the Ron Herman boutique on Melrose Avenue in Los Angeles.

She tugs at the garment, performs deep knee bends in front of the mirror, and even hoists her slender self on a display table to see whether the jeans dip too low in the back when she sits down.

"I'm dying," she groans. "Even in a (size) 28, I'm dying."

In the no-mercy world of fashion, skinny jeans are back. Some styles are essentially denim leggings — slim all over and tapered to just 5 inches across at the ankle. They should come with a warning label: Objects in mirror are even tighter than they appear.

The factors driving the trend are practical and, well, not. With the rising popularity of dress-casual boots, women want jeans that won't hide the $300 or more they have invested in their footwear.

Then there's pencil envy. Super-thin celebrities in super-thin britches — Kate Moss, Nicole Kidman and Keira Knightley — are helping to drive the trend.

"That was a lot of work," Korn says, before slamming down $150 for a pair of J Brand jeans to add to the mountain of denim she has back home in Philadelphia.

The trend is good news for manufacturers, especially in Los Angeles, the capital of premium jeans, where a pair may go for $400 or more. Jeans makers continually pray to the style gods for a new cut that will make people who are already buried in jeans want even more.

After introducing his J Brand straight-leg jeans last year, owner Jeff Rudes quickly followed with a narrower version — the "cigarette leg." Two months later, he was pushing a still slimmer "pencil leg." This March, his "super skinny" jeans arrived at the Ron Herman store.

When the manager heard they were coming, she asked, "How big is the knee?" Rudes recalled. "I said, 'It's tight.' She said, 'Make it hurt.' "

Melissa Fleis lusts after the denim jeans, which are laced with Lycra to ease the fit.

"I love these already," the Santa Monica resident said from inside the dressing room.

She bought two pairs. Total cost: $410.

Everyone wants a piece of the action, from high-end designers such as Dolce & Gabbana, which was on to the trend early, to Wal-Mart Stores Inc., which plans to start selling skinny jeans this summer for less than $19.

Whatever the price, millions of women will be less than thrilled that the oh-so-1980s look has resurfaced.

"I can't believe that we're in this trend again," sighed retail analyst Jennifer Black, who was wearing skinny jeans before her now-teenage son was born. Black plans to trot out the lone pair she still owns for what she hopes will be their last hurrah.

As for recycling jeans, she said, "I've heard you're really old if you do it three times."

Influential industry analyst Marshal Cohen — who has seen jean widths ebb and flow in 28 years of retail watching — fears that many women will buy into the hype when they should really hold back.

"Does anyone look in the mirror before they go out wearing some of these things?"

Men are being targeted, too. Los Angeles-based premium denim leader 7 for All Mankind, which began selling narrow jeans in Europe a year ago, introduced its Slimmy line for men last fall and now has a style that's even slimmer.

It was young men who helped relaunch the trend at Levi Strauss & Co.

About three years ago, the San Francisco company that dressed America in denim noticed a subculture of "young rock 'n' roll guys" wearing narrow pants at music festivals, spokeswoman Amy Jasmer said. To get the look they were after, some bought girls' pants — which they dubbed "GPs."

"They wanted them just so skinny and tight," she said.

Skinny jeans will represent a small percentage of the $12.5 billion-a-year U.S. jeans market, industry insiders predict. The boot cut will probably remain the favorite.

Denim vendors are thinking ahead.

In the mind of Jeffrey Lubell, chief executive at Los Angeles-based True Religion Apparel Inc., a mirrored ball is spinning. He's thinking huge bell bottoms ... John Travolta ... "Saturday Night Fever."

Consider this fair warning: "We're totally," he said, "going into disco."