(CNN) -- Even with the holiday travel crunch past, the issue of carry-on baggage remains a pressing topic for airlines and many travelers. After trying to crack down on carry-ons, several airlines are now making more room for the pesky baggage. But passengers will not necessarily be allowed to bring more on board, and not all classes or types of planes will get the upgrades.

Despite having one of the toughest carry-on policies in the industry, United Airlines is installing larger overhead luggage bins on its United Shuttle planes. After the bins are installed in the Shuttle planes, United will begin installing the larger bins on narrow-bodied Boeing 737 and 757 planes throughout its fleet.

United expects the new bins will be in place by the end of the year, but current carry-on rules will not change. The airline expects the bigger bins to reduce boarding delays.

United is not alone in opting for bigger bins: Delta Air Lines is installing bigger bins in its BusinessElite cabins on international flights.

Like United, Delta is maintaining its size restrictions and two-bag limit.

"There's a safety issue involved if too much hand baggage comes on board," said Fredric Reid, an executive vice president at Delta.

Continental Airlines has said all along that bigger bins are better. The airline already has the larger bins on most of its jets.

Continental has been waging a campaign against carry-on limits, retrofitting all its planes with larger overhead bins and opposing size templates on X-ray machines, such as the stainless steel templates that United is installing at 40 airports across the country.

"It's not OK to impose that same restriction on Continental's passengers, so this is really an issue about inconveniencing our customers and an issue about interfering with our customers," said Sarah Anthony, a spokeswoman for Continental.

One luggage trade group wants all the airlines to settle the issue by adopting the same carry-on standards. Last month, the Leather Goods Manufacturers of America petitioned the Federal Aviation Administration for standard carry-on guidelines for all carriers.

CNN Correspondent Stephanie Oswald contributed to this report.