I agree that what OP is describing doesn't sound like particularly friendly banter.
You've gotten a lot of great advice already. My advice? Double down, and/or turn it around on them.
- They say your car is small? Tell them yeah, you almost got that awesome Smart Car (or other even smaller car, or maybe unicycle) that you really wanted, but some jerk outbid you, so you ended up with this enormous, gas-guzzling tank. Bummer, huh?
- They call you cheap? Tell them damn straight you're cheap, which is why you're going to be sipping margaritas in Cabo while they're stuck in the rat race until they're old and gray.
- The gay thing? I'd probably say, "What the F do you care?" Or maybe, if you're both the same gender, with mock gentleness, "Sorry, friend, but I'm just not that into you."
- The skinny thing? Say, "I have never felt better in my life. Best thing I ever did, losing that weight."
When you own it and make a joke out of it, even if it secretly stings, you're telling them that they can't penetrate your armor there. And if they push it and it gets uglier, maybe those aren't people you really want to be spending your time with.
You can also turn it around:
"What's your fixation on big cars, anyway? That's kind of a weird fetish, but hey, whatever floats your boat."
"Hey, not all of us are trying to win the 'who can go into the deepest debt' game."
"Dude, you sure talk about being gay a lot. Is there something you want to tell me?"
"Hey, you just keep on loving your beer gut. I know how close you two are."