As the first poster noted, volume depends on the efficiency of the speaker being driven and how much air (ie; size of drivers)they move. Bigger drivers have a tendency to be perceived as louder.
However, they're other considerations also. Some amps allow for greater dynamical power (headroom.) For example, NAD's cheapie little 320BEE is rated at 50/ch into 8 ohms. However, it has about 6 db of headroom allowing it to operate similiar to a 150 watt amp on peaks of short duration and it will sound more powerful than an amp that has, say 1 db of headroom at the same power rating.
Other factors are how the amp interfaces with the speaker. Does the speaker have an low impedance with a high phase angle, etc. This example would cause an amp to have to be able to efficiently deliver current. If the power supply is not up to the task, then it want put out the power.
Remember, power ratings are a guideline only. They are rated into a load, usually 8 ohm dummy load at frequency's of 20 to 20 KHZ. The FTC mandates this. The problem is NO speaker is a flat resistance across the board. They may be 8 ohms at 1000HZ but 3.2 ohms at 60hz. The electrical phase angle changes with frequency. If the amp can't handle lower impedances and/or combinations of phase angles, then the power you thought you had is not there (see recievers below.)
Personally, unless it is a digital amp, I look at the heatsink sizes, cap values (power supply) and other features of a well built amp. Weight tells things sometimes. If you have a 100 watt amp without hardly any heatsinking, then you have to question the power. A good example is some of the BS ratings for hometheater receivers.
You actually load some of the 100w+ receivers into real world loads with all channels driven and that 100w translates into about "maybe" 30 per channel.
Car audio is the absolute worse for this non sense.