The specific behavior of the hash function (including the salt you chose). Choosing a different hash function (or a different salt) would result in a different

The specific behavior of the hash function (including the salt you chose). Choosing a different hash function (or a different salt) would result in a different breakdown into chunks. In principle, if your data were specially crafted to exploit the specific hash function (and salt) you could get an aberrant case like 1 million entries in a single b-tree node or a million b-tree nodes with just one entry. But unless you intentionally exploit the hash function the chance of this is vanishingly small.