A reference to an imposing thing means that it is magnificent, grand, massive, commanding, etc. by virtue of its size and/or forcefulness.
Francois Henri "Jack" LaLanne was an imposing American fitness, exercise, and nutritional expert, and motivational speaker who is sometimes still called "the godfather of fitness" and the "first fitness superhero".
LaLanne also gained an imposing recognition for his success as a bodybuilder, as well as for his feats of strength.
The imposing Arnold Schwarzenegger once exclaimed: "That Jack LaLanne's an animal!" after the 54-year-old LaLanne beat the then 21-year-old Schwarzenegger "badly" in an informal contest.