You're right; it's a little wacky. I'm not sure what the default is, but when I do custom foods, I put the serving size (oz., cups, 1/12 recipe, etc.) in parenthesis next to the name. Then I enter the values that correspond to that amount. If I eat a different size, I just adjust the serving when I log it in (.5, .75, etc.).
Hope that helps!
"If you drop an egg, you don't say, 'Oh, shoot' and drop the other 11, do you?"
-Source unknown, but obviously brilliant
Reached goal 4/16/2010...but kind of afraid to look these days