Which two out of the continuum of world then did you imply, and how did you select them? I don't see any way to select two specific worlds for which "relative thickness" would make sense. You can classify the worlds into "dead/not dead at a certain instance of time" groups whose measures you can then compare, of course. But how would you justify this aggregation with the statement that the worlds, once split, no longer interact? What mysterious process makes this aggregation meaningful? Even if you flinch away from this question, how do you select the time of the measurement? This time is slightly different in different worlds, even if it is predetermined "classically", so there is no clear "splitting begins now" moment.

It gets progressively worse and more hopeless as you dig deeper. How does this splitting propagate in spacetime? How do two spacelike-separated splits merge in just the right way to preserve only the spin-conserving worlds of the EPR experiment and not all possibilities? How do you account for the difference in the proper time between different worlds? Do different worlds share the same spacetime and for how long? Does it mean that they still interact gravitationally (spacetime curvature = gravity). What happens if the spacetime topology of some of the worlds changes, for example by collapsing a neutron star into a black hole? I can imagine that these questions can potentially be answered, but the naive MWI advocated by Eliezer does not deal with any of this.