I'm a guy like this. Same posture and same skinny frame.

https://marombacn.files.wordpress.com/2013/07/antestreino.jpg

I find it exceedingly difficult to get people to take me seriously as their boss. I have a strong voice but a weak body language. Often when I talk to people over the phone, they seem to have no problem with me taking the lead and giving them directions and telling them what to do. Then they see me in person and boom, it is all gone. I become the guy that is 'trying to act tough'. What can I do to change this and is lifting the only way?