Express shipping, which is often reliant on sending goods by air, has expanded rapidly during the pandemic as households turn to home deliveries. Purchases of manufactured products generally also have held up better than in previous downturns.

Money is chasing the higher freight rates. On Wednesday, Chilean carrier

LATAM Airlines,

which recently filed for Chapter 11 bankruptcy in the U.S., announced a big expansion of cargo operations. E-commerce giant

Amazon

is growing its fleet faster than planned before Covid-19, with a new focus on buying aircraft rather than leasing them. French container-shipping firm CMA CGM recently purchased four Airbus A330 jets to create an air-freight division. Shares in specialist cargo operators

Atlas Air

and

Air Transport Services Group

are up 100% and 29% since the start of the pandemic, respectively.

If the traditional economics of the freight business reassert themselves, though, investors could easily get burned.

The main reason air-cargo yields have jumped is because 30% of the commercial fleet remains grounded, dramatically reducing belly-hold space in passenger jets, which normally carry two-thirds of air cargo. This is much more a story about tight supply than rampant demand. Despite the jump in express shipping, the overall volume of air freight didn’t bounce back to pre-Covid levels until January, data by the International Air Transport Association showed this week.

While container-shipping firms are treating higher rates with due caution, air freight seems to have taken the windfall as a cue to permanently expand: 200 planes have joined the freighter fleet since May, according to data from analytics firm IBA. This sets the stage for a big capacity glut once most passenger planes start flying again, however slowly that happens.

The cargo fleet grows when companies either buy freighters or convert passenger jets. It’s not cheap: Price tags for used cargo planes have fallen by less than for commercial aircraft during the pandemic, and reconditioning a plane often cost north of $15 million.

Recent conversions have been led by the A330—traditionally an unpopular choice for medium-haul trips compared with the leaner Boeing 767—and newer versions of the 737. This suggests the post-Covid freighter fleet won’t just be larger, but also made up of bigger, more modern planes. Meanwhile, new passenger-jet models coming into the market like the 787 and the A350 will also have more cargo space than the ones being retired.

Expanding the fleet still makes sense for Amazon, which isn’t really focused on chasing short-term profits but rather on implementing a strategic shift in logistics that requires shedding part of its reliance on

FedEx

and

UPS.

Dedicated freight firms like CMA, however, may end up regretting opportunistic investments.

This much capacity growth only makes sense if e-commerce provides a much larger permanent boost to air-cargo demand than expected. More likely, many have confused pandemic resilience for long-term prospects.

Write to Jon Sindreu at jon.sindreu@wsj.com