A PCB I'm working with has three decoupling capacitors on an IC's Vdd pin – 0.01uF, 0.1uF, and 1uF. They are all in 0402 packages.
I understand that capacitors of multiple sizes are often used because parasitics tend to scale with size, but I was under the impression that this was generally because of different package sizes, not the actual capacitance values themselves.
I also understand that putting multiple capacitors in parallel will increase the overall capacitance while decreasing parasitic values, but I don't see why one would use varying capacitances for that rather than just using the largest value possible for each capacitor.
Is there any reason to use multiple decoupling capacitors with the same package but different capacitance values?
Here's a screenshot of the relevant portion of the circuit schematic (C1, C2, and C3 are all 0402 MLCCs):
Your initial impression is incorrect. Here is what the impedance of various capacitors in the same package looks like.
In order to get a sufficiency flat power supply impedance over a wide bandwidth, one needs to use a selection of different capacitors.