security of a hash algorithm is defined by its output size and the algorithm itself not by its input size.
Ok you hash a fixed 16 bit input with sha256 and ill find it in a second
that would be brute forcing which has nothing to do with security of SHA256. and the answer to your original question is "it is less secure if you use 16 bits instead of 512 bits input".
but since your original question is about 256 bit versus 512 bit input, brute force is not an option there since 256 bit is still big enough to make it impossible.