Maybe this has changed (or maybe it differs from place to place), but the fries in our Happy Meals (as of a few years ago) were the same size as the regular small fries.
I know this because our local McDonald's allowed us to have a fund-raiser. We worked behind the counter with them as volunteers on a slow night (I think it was a Wednesday), and they gave us a percentage of their sales (I think it was like 10%). It worked out great for both McDonalds and the school. We raised funds for playground equipment, Mcdonald's had huge sales on a typically slow night and the kids got a big kick out or seeing their teachers, principal and parents working at McD's
Anyway, back to the point here: One of my "jobs" was filling Happy Meal and french fry orders.
Jane