The output amplifier, drawing a lot of current, can cause the supply line to fluctuate. If this same supply line feeds a sensitive preamplifier, feedback could result, cause by the output signal getting back into the input of the preamp through the supply lines. So, the supply input to the preamp is decoupled. This cap may be 10 F to 100 F on an audio system or may be a 0.1 F disc ceramic cap in a digital system. The decoupling caps are smaller because they're being used in higher-impedance circuits where the current demands are smaller.
Bulk capacitance is just capacitance for capacitance's sake, the more the better.
It's value isn't as important as the fact that there's plenty of it. As long as there's a minimum amount of capacitance, all is well. While decoupling caps can do a similar job, they also take care of problems coming in the other direction. The output amplifier, drawing a lot of current, can cause the supply line to fluctuate. If this same supply line feeds a sensitive preamplifier, feedback could result, cause by the output signal getting back into the input of the preamp through the supply lines.