Say what you will, I doubt anybody will be able to convince me that it wouldn't be a LOT easier to knock up something good, be it big OR small, with a larger selection of pieces to draw from.
Even with a small mech, trying to build one that fits with a specific character's theme can require quite a huge selection to draw from in order to get all the specialized pieces you need AND in the right colors. I certainly couldn't build jackal76's mech in ANY color scheme, not having all those pieces in ANY color. I'd wager that having all those pieces, and in that specific color, represents either a LARGE pool that he's drawing from, or else a special order of parts made just for this contest.
I could build A mech from the pieces I have quite easily. Building one that looks like it belongs to a specific hero or villain (and one that I can knock up a reasonable minifig for) is the challenge, and one that is invariably made far easier by a larger collection.
I'm sorry if I sound like I'm complaining about the contest. I'm not, really. I think it is an awesome contest, and am eagerly awaiting seeing everybody's entries. I may even manage to knock something up myself. I'm really just lamenting the fact that I don't feel it is likely that I'll be able to field an entry I'll be happy with.
In general I DO feel like those with a bigger collection have an advantage in these contests, but that's kind of unavoidable unless you specify the same selection of parts to begin with, and I wouldn't want that to be the norm, as the "sky's the limit" format just makes for much more amazing results.