Well... Let's cover this bit by bit shall we?
He's apparently attracted to larger women. Nothing wrong with being a chubby chaser at all. However, apparently you're saying you don't fit this. Now, some additional questions that may sound racist, but tend to be more in line with societal thinking. Is he black? If so, were his past girlfriends large?
It sounds like one, he's just not that interested in you. He has some issues going on that make him incapable of being faithful. That's his fault, bad on him.
Now, if he's doing this pretty much behind your back (and it sounds as if he is) then he's cheating. If there were nothing to hide, and you were OK with it, then he'd be chatting w/ them openly with you, and you'd have some sort of relationship dynamic supporting this. However, you're not, and he's pretty much sneaking around behind your back with this. It's not quite like looking at porn, which is anonymous people. He's engaging in conversation with them, which means he's seeking out individuals, and not some passive media (Movies, pictures, smut stories...) to read for inspiration.
Perhaps he's unhappy because you wait on him hand and foot? Which, isn't your fault if he hasn't voiced this.
The BIGGEST question is why aren't you standing up for yourself and kicking this guy to the curb for being a creep? Seriously, there should be NO issue with you putting your foot down and saying, "OK I've had enough of this behavior and I'm not OK with it. Either you stop and I have permission to check on you since you have violated my trust with this, or we're done."
His behavior is bullshit, and it needs to end.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."