Davis was also an undersized front court player who was not a very good defender and only rebounded 3.5 rbg. That doesn't cut it. Dou is a better defender and rebounder, longer and more athletic. I think once he's had a chance to adapt, you will see much more upside in him than Davis.
The same can be said for Smith. Miller averaged 9 and rebounded OK but was less athletic and a major defensive liability and was 2nd on the team in turnovers even though he played moderate minutes. Again, the upside for Smith is greater than Millers. If you don't believe me, go look at Miller's numbers this year.