A lack of legroom has long been a criticism of air travel by passengers who are unable to fully extend their legs for hours on end. But the latest trends in seating could make flying even less comfortable, while helping to boost airline revenues.

Ultra-discount model
When it comes to offering a cheap fare, it's tough to beat Spirit Airlines (SAVE -3.80%). While taking on some of the strategies of more traditional discount airlines, Spirit adds its own touch through tight seats and numerous fees.

To keep unit costs down, Spirit packs as many passengers into a plane as possible, and this means some of the least legroom in the industry. At only 28 inches of pitch (the distance from a point on one seat to the equal point on the seat in front of it), Spirit's fleet offers far less space than the 30 or more inches of pitch found on most other airlines.

But Spirit operates on an ultra-low-cost model, so for many passengers, it's Spirit Airlines or the bus. No one's forcing you to fly Spirit, but its seating arrangement provides an example of the latest in cost-reduction strategies.

Slim fit
Do you think airline seats are too thick? Probably not. But every inch of extra seat thickness is another inch of space the airline can't put another seat into. Turns out, airlines have realized this, and slim-fitting seats are all the rage.

Southwest Airlines (LUV -6.96%) has joined this trend with plans for slim fit seats that enable the airline to add an entire new row of seating. While not on travelers' lists of  likes, the new seat designs are expected to add around $200 million in revenue to Southwest Airlines.

Delta Air Lines (DAL 4.05%) is also taking advantage of this opportunity with its own "slim-line" seats, which will allow the airline to add another 19 seats to its Boeing 757-200 aircraft. The move comes as Delta outfits aircraft for both greater revenue generation, as well as adding more electrical ports, and larger overhead bins. To do this, the airline expects to spend $770 million through the end of 2016 on aircraft upgrades.

More seats, more revenue
Air Canada (TSX: AC.B) is looking toward simply adding more seats as a way to build revenue and cut unit costs. Part of its expansion strategy has been through Air Canada rouge, a discount subsidiary that provides less legroom in exchange for a cheaper flight.

But mainline Air Canada is getting in on the action, too, with its high-density configurations on its Boeing 777 aircraft. Packing in 100 more seats than the standard configuration, the airline hopes to boost revenue with a relatively minor impact on costs. Passengers may be against this, but with airline seats tight across the industry, and passengers always seeking out lower fares, Air Canada's decision may well be the correct one.

Time for the upsell
Airline seats have been shrinking through a variety of revenue-generation and cost-reduction strategies. Amidst all the passenger frustration, airlines are helping to boost an entire new line of business: the ancillary revenue generating upsells.

In my next article, I'll take a look at how pinching passengers space is benefiting airlines through more ways than greater ticket revenues and lower unit costs.