G20 as opposed to G19 or G21 (world's largest economies) (finance, membership)

[yofie]

Why did the G20 (the world's 19 largest economies plus the European Union) evolve to become like that, and not (say) the G19 or G21 - because the finance ministers and so forth wanted a nice, round number of members? But on the other hand, the G20 was preceded first by the G22 and then, briefly in 1999, by the G33.

[Ghengis]

at #13 in GDP how come Spain doesn't have full membership? Sure they invite them to the meetings but I'm guessing without full membership they don't get a chair at the table and likely have to share the meeting minutes by looking over the shoulder of whoever happens to be sitting in front of them. Probably no box lunches for them either.