Many people do not know this, but the Sports Illustrated swimsuit models all pose completely naked, and it is only later that the tech wizards at SI airbrush on the body paint bikinis.
As proof take a look at this Kate Upton nude outtake from the 2014 SI swimsuit issue. As you can see she is butt naked in this original photo, and later they add the bikini and remove all the excess fat.
It is much simpler this way and safer from a legal standpoint. If someone had to actually go through the trouble of physically painting Kate Upton’s enormous and sinful nude body they would no doubt go on disability claiming carpal tunnel and post-traumatic stress, and probably file suit for being subjected to a hostile work environment.