Just keep your answer positive no matter how bad the reality was.

"I felt I was ready for a position with more responsibility"
"I like your company's direction in xyz and I feel there is more opportunity for growth"

If you can't find a positive spin, keep it at least neutral (I recently used my state's cost of living as an excuse, I left out the part about old job's horrible culture)

You may be totally justified in feeling slighted by the old company but never ever ever let on to the new employer. Venting is best left with friends