Size of Decoupling Capacitors in Digital Circuits
Parasitic inductance. Larger caps tend to come in large packages and larger packages have more parasitic inductance and therefore are not effective at high frequencies.

The limiting factor for decoupling low frequencies is capacitance, but the limiting factor for decoupling high frequencies is parasitic inductance. Ideally, you're better off using the tiniest possible package you can get, with the largest amount of capacitance available in that package, and then using a lot of them in parallel until to achieve the total capacitance you need to decouple those lower frequencies. If the  parasitic inductance of the smallest package you can get is still too high for the high frequencies you need to decouple, then you need go to PCBs with embedded, distributed capacitance.

ADDITION:

Inductors look a bit like big resistors to high frequencies and small resistors to low frequencies. The entire reason we need decoupling capacitors in the first place is because all wires and traces have inductance and sudden changes in current demand by digital circuits switching causes voltage drops or spikes via these inductors if the load current is flowing through them. This appears at the power pins of the digital circuits as voltage spikes or voltage droops which disrupt the circuit's proper function.

Similarly, the parasitic inductance in a capacitor is in series with the ideal capacitance and prevents high frequencies from flowing through them easily, which is required if the capacitor is going to properly decouple.