It seems unlikely that people are bigger on the inside than they are on the outside. But if you've ever spent any time with a young child, you'll know they frequently carry more excitement, creativity, power and perspective than they could logically have based on the size of their bodies and the length of their experience. Children are almost always bigger on the inside than they are on the outside.
Sadly, adults are sometimes smaller on the inside than they are on the outside. Although they look like they are competent and energetic and capable of generating visions of the future, they know themselves to be weak, blind, and tightly bound by invisible constraints. Although the rest of the world sees what they could do, they themselves are mesmerized by their limits.
No one can stay little for ever. We all grow up. A few people even manage to continue to get bigger on the inside even after their outsides stop growing. It's worth noticing them and wondering what they are doing different.