Fast-growing sales of the smallest cars are siphoning market share from compact sedans, auto industry experts say.

Sub-subcompacts "are the new hot darlings of the market," says George Peterson of industry consultant AutoPacific.

Sales of the minis doubled in the first quarter compared with a year ago, when some of the smallest models hadn't yet gone on sale.

"We're just starting to see more interest in that part of the market," says General Motors spokesman John McDonald. Sales of GM's tiny Chevrolet Aveo rose 44 percent in the first three months, while those of the next-size-up Chevy Cobalt were off 20 percent, Autodata reported.

Honda Fit, Toyota Yaris, Nissan Versa and Hyundai Accent were other tiny cars showing sales gains.

Meanwhile, sales of bigger compacts, including Nissan Sentra, Honda Civic, Ford Focus and Hyundai Elantra, tapered off during the first quarter. The category was off an average of 2.8 percent.

Dealers averaged a 42-day supply of the tiniest cars in the first quarter, 11 days less than for compacts, Power Information Network reports. A year ago, the relationship was reversed, with compacts in shorter supply.

Still, the minis are just getting started as a factor in the U.S. auto market. All the sales in the category totaled just slightly more than those of the second-biggest selling compact, Civic, during the first three months.

Some customers may be resisting compacts nowadays because they have slowly gotten bigger, says Alex Rosten, industry analyst for Edmund's AutoObserver.com. The 2007 Civic has the same basic headroom and legroom as a 1990 Honda Accord, traditionally the larger model.

Toyota, Honda and Nissan officials say their smallest models are not stealing sales from their mainstay compacts. "The cars are unique enough they are going to get their own buyers," says Nissan's John Schilling.

The customer profiles are different, says Toyota spokesman Bill Kwong. Yaris attracts younger, single buyers, while Corolla draws young couples.

Civic has seen slower growth this year because dealers are hustling to move more intermediate-size Accords before a new version arrives, says Honda spokesman Sage Marie.

And not all compact car sales are stumbling. Mazda3 sales are up nearly 47 percent for the first three months of the year.