If everyone else is using normal sized models, you might gain an advantage in LoS situations, as your units will have more options for being completely out of enemy LoS.
One thing to consider if you scale everything down (I know this as I did it to try to make a 'portable mordheim' at one time): The more you scale down, the more errors in measurement will effect gameplay. Getting 1/4 inch off in the full-sized game isn't that big of a deal, an error of 1/4 inch if you're using, say, round push-pins on corkboards (yeah, I did that once) and it becomes a huge deal. Scaling everything by half shouldn't cause too much trouble though, as half inches are still pretty big. For the corkboard thing I scaled it to 1 cm = 1 inch. Exact meansurements were a pain...