Up to this address width every address gets a slot number in one flat array: 2^16 ints is 256kB, which is always affordable. Above it the address space outruns what can be allocated and the memory is assumed to be occupied thinly.

Up to this address width an address still fits a uint32 and the trie can key on one. Above it addresses arrive as bigints and the lookup falls back to a Map - a memory with more than 4G words cannot be meaningfully simulated, so that path is a guard, not a design.

Compact once the tail is at least this long *and* at least as long as what is already indexed. Doubling like this makes the copying amortised O(1) a write. Waiting for a query instead, when there is nothing out of window to drop, was tried and is worse: a tail entry carries a slot number that an indexed one does not, so leaving writes in the tail costs 12 bytes each against 8, and on a hundred small RAMs that measured 19.9 MB against 11.8 MB with no speed to show for it.

Children per trie node. Four bits a level.