"Study reveals how having an older brother can make you shorter"
http://www.guardian.co.uk/science/2007/sep/13/genetics?gusrc=rss
Personally, I think being a little shorter would be an O.K. trade-off for being a little smarter.
From the experience of having a younger sibling, I wonder how much of the short stature is actually related to "getting less food" and how much of it is related to just constantly running it off trying to keep up with 'the big kids.' That could have something to do with the I.Q. Seems to me kids with older siblings want to do everything the older kids do. They might get a lot more verbal practice, etc., at a young age.
*edit* And yes, I do realize (belatedly) that I got the title wrong.