The idea that the universe consists of seemingly uncountable numbers of stars, each of which is another Sun and perhaps has planets of its own, is vastly more detail than the old heliocentric model of one Sun, a fixed number of planets, and stars being pinpricks in the tapestry surrounding the solar system. Nonetheless Occam's razor suggests that we accept the view that the stars are suns and we are just one planet among many billions upon billions, because although that universe requires vastly more detail to describe, it is governed by a simpler set of rules. Unifying heaven and earth under a single, simple set of physical laws, discovered by Newton, makes for a simpler scientific model, even if it introduces the possibility of the universe being vastly larger than we originally thought.

Similar situation with many-worlds. If MWI is right, then the "universe" if interpreted to include all reachable branches of the so-called multiverse, is vastly larger than we previously believed. But it is also simpler, in a strict Occam's razor sense, because it does away with the concept of collapse. Wave function collapse is an additional physical rule in the Copenhagen interpretation, whereas it's just an illusion and not an enumerated part of physical law in MWI.