You are talking about the PBS-1 and its clones, such as the Dead Air Wolverine and this Zastava.

AK's do not have the kind of standardization that many modern guns have, so AK silencers have to be made to accomodate their enormous amount of quirks, not the least of which is some thread patterns being as wide as 26mm. Yugo, Russian, Bulgarian, etc. have giant threads. Many other types are more normally sized, but the can also has to be able to change out mounts/adapters, etc. (Read the product manual for the Dead Air Wolverine, it is possibly the most complicated silencer on the market.) For all this to fit to fit inside a silencer of a relatively normal diameter, it has to flare out near the muzzle to accomodade the giant thread patterns as well as all the other extra parts that most rifle cans don't have.

There are a few other cans, like the Resiliant Putnik, which just keep the wide diameter through the length of the silencer. The increase in diameter down the entire tube allows the silencer to be shorter without sacrificing internal volume.