My understanding, which may be woefully wrong, was that in a parlaimentary system the parties each present a slate of candidates and the electorate voted for a party, not for a candidate. Then if, say the election turned out 40% Labour, 30% Tory, 20% WNP, and 10% some small party I've never heard of (SSPINHO) and parlaiment had 100 seats, then the top 40 from the labour slate would be seated, the top 30 from tory, the top 20 from WNP, and the top 10 from SSPINHO. (obviously a simplification of the numbers but I think you should understand what I am getting at).
I thought that this was why your system tended to promote larger numbers of parites, becasue you only need to get about 5-10% of the overall vote to get seated. Whereas in the US congressional seats are for specific districts and one needs to get at least 51% of that district to get seated. This tends to promote just two parties strong enough to get over 51% of enough districts to be seated.
It seems obvious from what I have heard so far that this is not completely accurate, so please correct where I have erred