Thanks for posting your query.
Small for date would mean, the weight of the baby is not up to expected standards for that particular time of pregnancy
There would be many causes for this which would be maternal, fetal or placental.
It does not actually go with the diet.
The cause like if any placental abnormality or pre-eclampsia would need to be corrected for this.
Hope I have answered your query.
Wishing you good health.