Why is the output volume lower on a 1 transistor preamp
The schematic shows that this is a voltage amplification stage with poor current sourcing ability (that is, not a power amplifier by any stretch). Its output impedance is essentially that of the load resistor R3, and that is 10K. (That is a property of common-emitter amplifier circuits: the load resistor determines the output impedance.)  So it is not going to work well into loads whose impedance is a three digit figure, let alone two digit or single.

The author of the circuit knows this, which is why he calls it a preamp. 

If you hook it up to headphones, it will not compete with the amplifier in your portable device, which is properly designed to drive their low (typically 32 ohm) impedance.

The earbuds may be "small", but they are hard to drive due to their low impedance: as a load, they are "big".

When an amplifier that has a high output impedance is asked to drive a low impedance load, then most of the signal voltage is dropped across its own output impedance, and only a small part of the voltage is bridged to the load. The output and input impedance essentially form a voltage divider.

Explore using different loads with this circuit. For instance compare the iPod going directly into a PC's audio input, versus through this circuit.