As we see, weapons' might is becoming incredibly great, no matter how strong you are, you are nothing compared to the weapons and you can't drive them only by your strength. So being big just means you're nothing but a huge target. As for the resources and rooms( of vehicles or habitations) are all limited, being small means frugal and indirectly means being richer. Moreover i've heard that the more cells one has the more complicated his body system gets to be, thus the more unpredictable illness will happen easily. Afterall I think it's better to be smaller for us human beings. The only question is how small we are going to be is the best? As small as an ant?