B+Tree leaf nodes contain keys and values. In kvlds (which is the core B+Tree component of kivaloo), the code aims to keep nodes at least 2/3 full, in order to keep the tree approximately balanced. If a key-value pair can take more than 1/3 of a page, you could have a situation where a node is less than 2/3 of a page but adding another key-value pair would take it above the maximum allowed size.

This restriction could be relaxed, e.g. to require only that internal nodes are at least 2/3 full, in which case for the small key / large value case you could have pages which are barely larger than the largest value. I didn't bother doing that since it wasn't relevant to my usage.