You gain based on your natural un-shifted stats, not your forms. Doesn't matter if your form has 25 or 3 con.

Fully ignore your form stats and it'll make more sense.


CONSTITUTION
Constitution: Affects how much health you gain when you level, how quickly you
regenerate it after being injured, and your resistance to
certain maledictions. Having the strongest constitution you can
achieve early on will help you throughout your life. Again,
shapeshifters gain based on their natural un-shifted stats, not
the constitution of their form. For more information on Con,
see HELP DEATH.