Django validation is mostly application level validation and not validation at DB level. Also Model validation is not run automatically on save/create of the model. If you want to validate your values at certain time in your code then you need to do it manually.
For example:
from django.core.exceptions import ValidationError
form app.models import MyModel
instance = MyModel(name="Some Name", size=15)
try:
instance.full_clean()
except ValidationError:
# Do something when validation is not passing
else:
# Validation is ok we will save the instance
instance.save()
More info you can see at django’s documentation https://docs.djangoproject.com/en/1.10/ref/models/instances/#validating-objects
In administration it works automatically because all model forms (ModelForm) will run model validation process alongside form validation.
If you need to validate data because it is coming from untrusted source (user input) you need to use ModelForms and save the model only when the form is valid.