Comment by hyc_symas

LMDB and SQLite are not directly comparable. LMDB is a transactional B+tree-based key/value store. SQLite is an implementation of a transactional SQL data model on top of a B+tree-based key/value store, so it is logically at least one abstraction layer higher than LMDB. (Key/value stores underlie pretty much all of the other data models you'll ever use.)

That aside - LMDB is not just smaller, faster, and more reliable than SQLite, it is also smaller/faster/more reliable than SQLite's own B+tree implementation, and SQLite can be patched to use LMDB instead of its own B+tree code, resulting in a smaller/faster footprint for SQLite itself.

Proof of concept was done here https://github.com/LMDB/sqlightning

A new team has picked this up and carried it forward https://github.com/LumoSQL/LumoSQL

Generally, unless your application has fairly simple data storage needs, it's better to use some other data model built on top of LMDB than to use it (or any K/V store) directly. (But if building data storage servers and implementing higher level data models is your thing, then you'd most likely be building directly on top of LMDB.)