The old Subdivide button was behaving as if subdivision modifier was
applied on top of the multires. This was the source of shrinkage since
the behavior of the limit surface: limit surface of a sparse point
from another limit surface makes final result appear smaller.

The new behavior is based on propagating delta against base mesh's
limit surface to the top level. Effectively, this is as if we've
sculpted on old top level and then propagated to the new top level.