diff --git a/libraries/liblmdb/lmdb.h b/libraries/liblmdb/lmdb.h
index 9c21cdc..e4d55c8 100644
--- a/libraries/liblmdb/lmdb.h
+++ b/libraries/liblmdb/lmdb.h
@@ -574,6 +574,9 @@ int  mdb_env_get_path(MDB_env *env, const char **path);
 	 * of the database. The value should be chosen as large as possible,
 	 * to accommodate future growth of the database.
 	 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
+	 * The size may be changed by closing and reopening the environment.
+	 * Any attempt to set a size smaller than the space already consumed
+	 * by the environment will be silently changed to the current size of the used space.
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[in] size The size in bytes
 	 * @return A non-zero error value on failure and 0 on success. Some possible
diff --git a/libraries/liblmdb/mdb.c b/libraries/liblmdb/mdb.c
index 4fe330b..b25c372 100644
--- a/libraries/liblmdb/mdb.c
+++ b/libraries/liblmdb/mdb.c
@@ -2722,11 +2722,22 @@ mdb_env_open2(MDB_env *env)
 			return i;
 		DPUTS("new mdbenv");
 		newenv = 1;
-		meta.mm_mapsize = env->me_mapsize > DEFAULT_MAPSIZE ? env->me_mapsize : DEFAULT_MAPSIZE;
 	}
 
-	if (env->me_mapsize < meta.mm_mapsize)
-		env->me_mapsize = meta.mm_mapsize;
+	/* Was a mapsize configured? */
+	if (!env->me_mapsize) {
+		/* If this is a new environment, take the default,
+		 * else use the size recorded in the existing env.
+		 */
+		env->me_mapsize = newenv ? DEFAULT_MAPSIZE : meta.mm_mapsize;
+	} else if (env->me_mapsize < meta.mm_mapsize) {
+		/* If the configured size is smaller, make sure it's
+		 * still big enough. Silently round up to minimum if not.
+		 */
+		size_t minsize = (meta.mm_last_pg + 1) * meta.mm_psize;
+		if (env->me_mapsize < minsize)
+			env->me_mapsize = minsize;
+	}
 
 #ifdef _WIN32
 	{