Smaller packages have different resonance points than larger packages. Larger packages also have higher lead inductances(you'll need to think about through hole packages).
Smaller packages are always better for high speed since they reduce the length a signal has to travel. As you know, for high speed design, the longer the length the more problems one has. This is why FGPA's can operate so fast even with many paths because the paths are all crammed on such a small area.
There is a good analysis of smd package sizes online somewhere(I don't have the link but saw it ones). It talks about why one should use both large and small sizes for bypassing which has to do with resonance. Decoupling is a different story though. It all depends on what kind of signal you want to decouple.
Smaller generally is better simply because it allows one to reduce the signal path. This is always good. It is not always the case the smaller is better though(you end up with other problems such as crosstalk).
Note that when you parallel things you may reduce some factors you will also increase others. If you parallel resistors you might reduce their resistance but you increase their capacitance. It may be a higher capacitance than if you just used one resistor with the combined resistance in the first place.
When dealing with decoupling capacitors another factor is leakage. This paralleling capacitors increase the leakage. This is usually pretty bad for decoupling because you are not as decoupled as you may want.