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On a recent trip to the Gap, Trisha Hart and her mom both happily left the store with clothing in a size smaller than they normally wear. But they haven't been on a mother-daughter diet.

"If anything I keep getting fatter," Hart said.

So why the shrunken sizes? According to fashion experts — and the fitting-room accounts of female shoppers — designers are cutting women's clothing bigger in an effort to cash in on womanly vanity.

"It's a very common practice," said Tamara Albu, fashion design coordinator at Parsons School of Design. "Designers make women feel they're a size 4 and they make a sale. It's a marketing trick."

Albu said Ralph Lauren, Tahari, Betsy Johnson, Cynthia Rowley, Nicole Miller, Banana Republic and the Gap are all known for deflating their sizes.

Her colleague Pam Klein, chair of the associate degree program at Parsons, agreed that fitting-room fraud is everywhere.

"All companies and designers that are targeting the Sex and the City customer have a similar strategy," Klein said.

While the alleged designer deception may not be fooling anyone, it nevertheless appears to be working. Jen Muzio, 26, said she likes shopping at Express because she is a 6 there instead of her normal size 8.

"I get all psyched up, even though I know I'll be an 8 next door," she admitted.

Michelle Albera, 24, felt the same way. "If it's larger than an 8, I won't buy it even if I want it," she said. Both Albera and Muzio avoid Old Navy, where they often have to go a size up to find clothing that fits.

But a spokesperson for Gap Inc., the parent company of Gap, Old Navy and Banana Republic, denied that "vanity sizing" is going on at their stores.

"Our sizes haven't changed in 10 years — even with the introduction of extra-small and size 0," spokesperson Stacy MacClean said.

However, a size 8 at the Gap is not the same as the 8 at Old Navy and Banana Republic —and this, according to Grace magazine editor Ceslie Armstrong, is the key to understanding "vanity sizing."

"It's a branding thing. Each designer is looking to attract women with a certain silhouette,"  Armstrong said. "Women are very loyal. If they find a line that works for them, they'll stay there."

MacClean confirmed that Gap Inc.'s sizes vary with their targeted demographics — for example, Old Navy is geared toward kids and moms, Banana Republic toward professionals.

She also said that while sizes at the Gap haven't changed, fits and silhouettes do fluctuate with fashion.

But amid all this clothing confusion, one thing remains a fact: Some designers work better on certain body types than others. So Armstrong's advice is to find out which ones work for you and to stick with them.

"Stop obsessing over size and shop for your silhouette," she said.