THE GLACIAL PACE at which menswear evolves leaves it short on “Eureka!” moments, but designer Hedi Slimane ’s debut collection of ultra-skinny suits at Dior Homme in February 2001 is widely considered one of them.

Hip-hop artist Shaggy sat atop the Billboard Hot 100 and boot-cut denim was still king when Mr. Slimane unveiled his sharp-tailored rebuke to the beefy “power suit” that had dominated menswear since the late 1980s.

Although the fashion press greeted Mr. Slimane’s streamlined silhouette with elation, neither the industry at large nor the average guy was ready for it. Not yet, anyway. “When he did the first show, everybody was saying, Oh, what is this?” said L’Uomo Vogue fashion editor Robert Rabensteiner.

But eventually, the once unthinkably trim ideal rippled out into the mass market, helped along by Belgian designer Raf Simons and American talent Thom Browne —not to mention the neatly cosmopolitan 1960s suits in which Don Draper lied his way through “Mad Men.” Today, “slim fit” is ubiquitous—a standard option at even old-line, salt-of-the-earth retailers like L.L. Bean, and the default silhouette at most European houses—and high-end designers seem to be asking: Where to next?

If the current fall collections are any indication, the way forward is to let it all out. After years in exile, a softer, slouchier (but not sloppy) silhouette has billowed its way back into fashion. Models in blanket-sized outerwear strode runways at Ermenegildo Zegna Couture, Berluti and Louis Vuitton. Even Dior Homme, now designed by Kris Van Assche, showed boxy, long coats that evoked the startling “big suit” David Byrne wore in the 1984 concert film “Stop Making Sense.” Meanwhile, baggy trousers, often styled with sneakers, were made luxe-cool by designers like Pablo Coppola, the sharp Spaniard re-envisioning the Swiss brand Bally, and Alexandre Mattiussi of young French label Ami.

In many ways, this movement is simply a natural swing of the fashion pendulum. “For the past 10 years there’s been this overarching aesthetic of slim, structured, buttoned-up tailoring, but I think we’re seeing now that guys are ready for a little bit of a change,” said Jeff Hilliard of Hong Kong-based men’s emporium the Armoury, which opened an outpost in Manhattan’s Tribeca neighborhood last year.

Before we go any further, please note that the new direction isn’t an excuse for ill-fitting clothing. Men who are still wearing suits with linebacker shoulder pads and 10-inch-wide trouser legs: You’re not suddenly fashionable. “That’s where we draw a distinction,” said Mr. Hilliard. “We’ve always appreciated a soft, comfortable silhouette, but we’re champions of clothes that fit. But fit doesn’t have to mean very tight or very loose. Fit comes down to what makes a person look better.”

Balance and proportion are key to getting the less-strict look right; not everything you wear should billow. “For me, the relaxed silhouette looks best when it’s just one item,” said Ami’s Mr. Mattiussi. “Either trousers or outerwear, not both.” In his fall show, he paired baggy trousers, tapered a bit to get a just-so drape over the shoe, with fitted bomber jackets and close-shouldered coats.

Stefano Pilati, head of design at Ermenegildo Zegna—a proponent of looser tailoring since he masterminded Yves Saint Laurent’s menswear in the mid-2000s—endorses the same strategy, romantically calling it “an exercise of style where formality and leisure breathe the same languid allure.”

Translation for those who don’t speak Pilati-ese: A roomy and rounded coat that might have belonged to someone’s grandfather draped over a fitted turtleneck sweater and slouchy-but-still-slim pants, as seen in Zegna’s fall collection.

The formula of voluminous coat over a trimmer ensemble is probably the easiest way to dip into this trend. There are many coats to choose from, it’s much easier to slip the new models over a suit, and the level of commitment is lower: You can always take a coat off.

“Comfort,” a word that historically has a tenuous relationship with high fashion, comes up a lot in discussions of the current collections. “Men now seek comfort first and are rarely just interested in appearance,” said Mr. Mattiussi. “Having a light, comfortable suit or coat will definitely help make your day easier.” These clothes are more forgiving than the skinny silhouette. You may even be able to have dessert and an extra drink at dinner without worry. ( Karl Lagerfeld famously lost over 90 pounds to wear Mr. Slimane’s wares.)

The looser look is indicative of a more casual attitude—a sign that menswear’s obsession with dressing up to the nines is beginning to ebb. That’s territory that Mr. Pilati began to push into last spring with his debut collection for Zegna which featured “broken suits” of mismatched jackets and pants and unexpected sporty details like long T-shirt sleeves rolled back over suit jackets.

Fashion editor Mr. Rabensteiner has been observing the slow move away from classical tailoring for a few seasons. He recently embraced a more casual silhouette himself, reserving his more tailored “stiff” clothing for formal occasions. “In the last seven years or so, everyone was trying to be really elegant; every man in the fashion business was wearing suits,” he said. “Now I don’t want to dress in a double-breasted suit to go to work anymore. I want to be easy and comfortable, dressed for normal life.”

Though there’s no need to trash your slim suits yet, looser silhouettes also figured into the 2015 spring collections. And this summer, J. Crew—the brand that brought skinny to the masses with its Ludlow suit—introduced the Crosby, a style with more breathing room. As to whether bagginess will become menswear’s new normal? Let’s regroup in five years.