I think his lies tend to be refutations of things that make him feel uncomfortable. A general "Chris is lying" conversation:

"Chris, you're fat and out of shape!"

"No I'm not! I eat good food and exercise every day"

"You don't exercise, you can't even do a single pull-up"

"Well. Hmmm...yeah. But I eat the Good Food every day!"

"No. You eat at McDonald's every day."

"Hmmm. Yeah. Well, somedays I do not. Yesterday I shoved vegetables down my throat!"

"No yesterday I saw you at McDonald's. You were eating four Big Macs. And dipping them in a bucket of Crisco like it was barbecue sauce."

"This hurtful truth level trolling is causing me much emotions and distress. It starting as a pain in my chest and extending down my left arm...

"

"Chris, go to the hospital. Get to the ER now. Please!"

"Hospitals are expensive!!"
OK so now that I've amused myself... I really think that for the most part when Chris lies, he's doing it because he's trying to evade emotional discomfort, or guilt. Things like the mechanic cracked the engine block when it was really Chris. Or any discussion about his health. He can (and usually is) almost immediately called on his lies, at which point he retreats to another lie. He kind of always stays in that comfort zone created by lies. I don't think a lot of his lies are malicious, they're simply an immature reaction to negative feelings.
EDIT: LOL ok tag me late. This shit took forever to write.