[font="Trebuchet MS"][size="3"] Hi,
Does the capacity (in pax) of a plane influence the nr of pax which will fly with the
respective flight? For example, if I fly with a plane of a specific configuration which
has 30 pax capacity and I get 20 pax per flight, if I bring a 20 pax capacity plane, with
the exact same configuration (except the pax capacity) will it get 100% load, providing
that nothing else is changed (competition etc.)?
Thank you!
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No, size of the aircraft is not important to AS passengers.
Off topic: Now I don’t want to be rude, but would you please stop writing everything in large font? It’s a bit annoying…
Short answer: yes.
Long answer: Different planes have different ratings in the eyes of passengers. You 20-passenger plane might be rated lower than your 30-passenger plane and therefore get fewer passengers. The reverse might also be true: a smaller plane might be rated better than a big one. But this won’t help much in this case since all 20 seats will be full anyways. Assuming the two planes are rated equally, you should get a 100% load with the smaller plane.
He asked about size itself. Rating includes more than just size, but fine, good thing you explained it anyway. 
Sorry, I did it because I thought it’s easier to read. Look, if just 2 more are telling me the same, I’ll stop doing it! 
Here is 1 more.
If nothing else changes then in theory yes. However there’s a good chance if you change an aircraft you’ll change something else even if it’s unintentional (flight attendant to pax ratio, aircraft rating etc.). Your best bet is just some trial and error.