(The pay-off is worth it...trust me)
ERNIE: Now, as you can see, I am pretty tall.
Hmmm...can we? We don't have a very good frame of reference. We can't even see your feet!
Originally, Big Bird was conceived to be, and was performed as, a rather imbecilic character...a "big, dumb animal," if you will. Later on, they decided he should be more of an overgrown kid--naive but not unintelligent. This sketch seems to date from the transitional period, which explains why Big Bird's voice is so weird.
Either that or he's suffering from a case of bacterial sufferosis. Come to think about it, his "hair" is kind of funny too! Oh no!
(Look, I've waited 30 years to be able to make Fraggle Rock allusions and now I'm going to have my fun. And eat it too. Or...something.)