This pixelization is partially enlargement (the two images are of different sizes) but L_D is absolutely correct -- know your codecs. I always render uncompressed avi at slightly larger (1 to 5%) than intended viewing size and then reduce afterwards.

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)