I agree with Jennifer. It is something he is probably ashamed of, as it's not socially acceptable to be obese, much less enjoy supersized BBWs... But I truly do understand that some men can appreciate the tiny girls to the bariatric beauties and everything in between... my man is one of them.
Just let him know you happened upon the site, not checking up on him or anything (if he were really afraid of anyone finding out he'd probably have cleared his browser hx) and were curious about it. You may also find that if you tell him a fetish or fantasy of yours that is kind of embarassing to you, that your intimacy will improve... honest talking, especially when it's something that really makes us open up and feel vunerable, does that.
Don't be too worried about it, or judgemental about it. There are different tastes for everyone, and some people like vanilla, chocolate, pistachio, and rocky road. ;)