What do I have to do to get it the same?

Why is your body shape different sitting down than standing up I love my shape when I’m sitting down but when I’m standing up everything disappears & I look very small with a chubby belly😅. What would help me have the same shape sitting down & standing up.?

I hope this isn’t a dumb question it’s just really making me insecure..

Ik some people who look the exact same way sitting & standing up tho🙁