Changing filesystem resize patterns

I would think there is quite a big difference.

In the big address space case every data structure can be laid out in sparse pattern. But initially there is clamp on the allocator on how much of journal/tree can consume for each data structure.
When growing the allocator limit is raised, but there is no need to move any data around. The data structures are all valid as is and 'continuous' to the allocated space.

With the current growing logic either the data structure space needs to be split to regions (increases complexity and slows down) or they need to be relocated (dangerous/slow operation).