Imposing is an adjective reserved for those things that are impressive in the sense that they are large or serious, as in the US Capitol Building or even, say, a professional wrestler.
When you look at the word imposing you see the word pose, which is useful in remembering what it means. A person who is imposing might strike a pose because they feel they have a lot to show. In fact, imposing is derived from the French verb imposer, which means "to inflict." Its French roots may inspire you to envision the enormous and expensive wigs worn by members of the French court. The larger the wig, the more imposing it was. And all the more reason to strike a pose!