Well link please then. Last I heard it was everyone starts at medium then can unlock heavy at 5 and 7 for light. You get a trial period with the body type but it was permanent. Could be changed so much is this close to release.
From http://www.fragworld.org/frag/community-blogs/brink-info-pt-3-weapons-and-customization.html:
"It has been confirmed that when you’re between missions, you can change your body type via the appearance customization options, however, DURING missions, you cannot change your body or appearance."
That is of course older information, so who knows what the current situation is. Sounds like you can switch body type, though.