I spent well over 100 hours on base Rise and didn't even come close to completing all its content.
Base Rise has the same number of maps as base World, more monsters, over 100 quests, and hundreds of weapons and armor sets to craft, all of which add up to potentially thousands of hours of content.
I'd hardly call that barebones.
I think the dimensions of the maps matter. The number of quests too. On average onHow Long to beat people who plays for leisure spend 37 hours more to beat World (78 hours) than Rise (41 hours) to beat the main story. I played neither more than the main story, but I always felt like World has more quests I was letting ignored compared to Rise too
I don't call Rise barebones either, it has enough content for a 60 USD purchase, but I think it's about half or less than World
Depends what you define as "beating" it.
Monster Hunter players typically don't just try to reach the end credits, the point of the game is to invest long term in the crafting and mastering the hunt, which takes hundreds if not thousands of hours. It's not the kind of game usually played in a "leisurely" manner due its high difficulty and the commitment it asks of the player.
I suspect the main time differential between the two comes down to Rise requiring less grinding to craft weapons or armor, a deliberate design decision by the developers. Rise is more streamlined, but not because it has less content.