Mind you I wouldn't call pumping iron despite your screaming muscles a particularly efficient way to improve your ability to speak in public despite your screaming brain. But it certainly helps. Some relevant mediating factors:

• Improved willpower and self control.
• Decreased salience of psychological distress - you can feel the discomfort without it needing to control or define you.
• Increased testosterone levels promote social risk-taking.
• Physical conditioning increases self esteem - your perception of your own status. The instinct to not draw public attention - and not place yourself above your station - is intrinsically linked to your relative status levels.
• Exercise changes posture. Even things as simple as standing differently change how difficult public speaking is!