Standard Bluetooth Mesh works by having every relay device send messages to other mesh devices in range. This is known as managed flooding because steps are taken to not relay the same message twice and not relay too far (limited by TTL value and maximum of 127). Also, for congested networks, it possible to send more than once to mitigate the affect of a message being lost due to collisions when two devices transmit (radio) at the same time. For this flooding scheme, you can imagine there can be significant relaying for just one message.
Managed flooding has the advantage that no centralised routing functionality is required, the failure of which would cripple a network. Flooding also implicitly provides multiple routes from A to B. Messages can still be directed to specific nodes. A node just examines the destination address to see if the message is for itself.
Bluetooth mesh is based on Bluetooth 4.0. Bluetooth 4.0 was previously designed for power efficient transfer of small pieces of information, relatively slowly. The Bluetooth packet is only 33 bytes and once packets are used for mesh, the available space for solution specific data goes down to 12 or 16 bytes depending on the setup. Multiple packets are used where data sizes need to be larger.
The mesh managed flooding together with the slow (power efficient) transfer mechanism means that throughput is relatively low. In practice, throughput of a mesh is only or the order of 2400 bits per sec. That’s bits not bytes! This means that Bluetooth Mesh is suited to sending small amounts of data that don’t change very often.