The technology didn't change. The artist just seems to be different. The big differences are:
- the sequel sprites are larger, so it's easier to get more detail in, and the anatomy can be less ambiguous.
- the sequel sprites have better use of Batman's cape for adding oomph to the animation, the original was rather stiff. The animation seems better in general.
- the first game had lighter colours overall to help Batman stand out against the dark environments and could avoid using outlines and instead have some external AA. The sequel had both dark and light-ish environments, necessitating a sprite that could stand out against all of them, hence the greater reliance on outlines. Because bright colours were no longer helpful with keeping Batman visible, they went for more "natural" colours for him, using the deep blues.
IMHO the original game looks much better (at least in stills; as I mentioned, the animation is stiffer), so I guess it's just a matter of taste xP The original using external AA because of the predictably dark environments is clever, and the art is less noisy, making it easier to read Batman's pose. The sequel's art, in comparison, has better animation, but the pixel art, though by no means bad, isn't as clever or clean.
