My big dog is a gentle giant.
gentle giant
idiomA person or animal of a great size or strength, yet who is of a friendly nature and not aggressive or threatening.
Origin
first recorded 19th c.From gentle + giant. First use appears c. 1807 in a translated work. See cite below.
Even though he looks intimidating, the new security guard is a gentle giant who loves helping people.
Despite his imposing stature and history as a champion wrestler, he remained a gentle giant who treated everyone with quiet kindness.