CAM plants are not tall because:
1. tall CAM plants would be unable to move water and minerals to the top of the plant during the day.
2. tall CAM plants would be unable to supply sufficient sucrose for active transport of minerals into the roots during the day or night.
3. night time transpiration will put a lot of demand for water on tall CAM plants.
4. tall CAM plants have a very negative water potential in roots in night as transpiration occurs rapidly in night.