From this we see:bigger battle is better
Maybe it's an indication that people who have made the EFFORT to buy all those figs (and it takes effort, to be sure), are the more devoted, or possibly the older LEGO fans: the ones with more experience and love to lavish on a creation.
For example, I think that Jens' Snake Knights vs the Orks would have still been a great battle even if it was 1/3 as large. He got first because of the DETAIL he put into it.
The chance for you to win is not so much in the pieces you have, although a certain minimum is required, but in the experience and love put forth as skill in your building.