On the SD forums, somebody asked this:
It is a well known fact that there will be Heavy, Med and Light body for every class but apparently they arent implemented yet.
Why is it so? Will it be dropped?
To which a dev replied:
They are implemented and have been for a long time, we just gave people the medium bodytype for the recent game shows as there's already a lot to absorb in a limited time.
So that being said, according to the dev, everyone who played the demo, played as a Medium. Now, look at this gameplay footage (yes, I know it's crappy) and pay attention to his health bar in the upper left corner. http://gamevideos.1up.com/video/id/30831
Whenever he takes damage, he's seems to be taking a lot, and getting killed pretty fast. This brings up a couple points:
1. If a Medium is taking that much damage, then a Light must REALLY have crappy health, even if every shot that hit him in the video was a headshot.
2. If a Medium takes that much damage, Heavies might not be the juggernauts some people perceive them as.
3. based off the damage scales displayed in the video, miniguns, regardless of their low damage, are going to tear Light players apart. I'm not boasting Heavy weapons here, I'm pointing out that players are gonna really need to work together and play to their class and body type's advantage.
A Light player can do a lot of the things mentioned above, IF you're skilled enough to pull them off. I predict a lot of players that initially wanna go Light, will be disappointed or simply won't have what it takes to really utilize the body type, and will opt for a Medium or even Heavy instead.
I don't see a lot of Heavies "jumping ship" and favoring another body, because everything the Light is lacking the Heavy has and everything the Light has Heavy players don't really care for, whereas EVERYONE likes having extra health.
Mediums being the "safe" class, could go either way, which is initially part of their purpose in the game. You start off being able to only play Medium, then you unlock Light and Heavy and begin to "fine tune" your playstyle.