As for your second question, I'm confused about whether you're embarrassed to be seen with your fat friends or whether your main worry is that Jim and others will crack jokes at your slim friends, making them self-conscious. If the former is the real issue, don't be so shallow. If the latter is the concern, I wouldn't worry too much; most people love getting called skinny!
Regarding your last question, I don't see why you have to invite your fat friends to go play tennis when you know they're not interested. A therapist once offered me this excellent piece of advice: "You don't have to tell everyone everything." Well, you don't have to invite everyone everywhere, either.
Friend or Foe
Dear Friend or Foe,
My husband and I regularly hang out with another young couple, "Mike" and "Nicole," and we have always had a great time. Last year, they welcomed their first child into their family, and we couldn't have been happier for them. We even threw them a baby shower in our home. Since the baby arrived, however, Nicole has turned into one of those overprotective, paranoid sorts of mothers. I understand that it's her first child, and she's worried about something happening to him. But it's getting out of hand. She freaks out when a box of tissues falls on him or the family dog bumps into him as it walks by. Also, we're no longer allowed to hang out with them in the evenings after the baby has gone to sleep. So we only see them for lunch on the weekends.
Even worse, while we're together, all Nicole wants to talk about are the stresses and woes of parenthood. What she doesn't see is that she's making it stressful for herself by worrying so much over tiny things. My husband and I don't know how to respond. ("That's no big deal," definitely doesn't work.) So we find ourselves focusing our conversations on Mike, effectively leaving out Nicole, who no longer seems interested in what's going on in the world. Is there a way to say to them, and especially to Nicole, "We liked you better before you had your baby?"
Missing Our Friend's Sanity
You may have liked Nicole more before she had Tiny Tim. But telling her so isn't going to get you anywhere, since, in all likelihood, the kid isn't going anywhere for the next 18 years. The good news is that Tiny Tim is on his way to becoming Not So Tiny Tim. Ideally, as the child gains independence and continues to bang into every piece of furniture in the house—and continues to live—his mommy will also begin to see that every falling Kleenex is not going to create a large dent in his head. A lot of new mothers get freaked about the death thing, becoming convinced that their babies will stop breathing during the night or otherwise expire on their watch. It's heavy stuff, and it takes some parents more time than others to live with the fact that we can only control what we can control.
Moreover, Nicole might be suffering from post-partum depression and not realize it. If you get her at a quiet moment, tell her she seems really stressed and ask her if she's finding motherhood "fun" at all so far? Make you a bet she answers in the negative. I'd give Nicole some time to regain her personality and interest in the outside world before you write her off. If in a year from now she's still treating you guys like uncovered electrical sockets, feel free to turn off the switch. But I bet she recovers her old self sooner than you'd expect. In the meantime, by all means direct your conversation to her husband, who's obviously taking the transition better. Or tell them lunch doesn't work and see whether they get the message.
Friend or Foe