I'm pretty sure fat people who have that extra strength have it due to them heaving their Gunts around in search of their next bite to eat, and even then that's got its limits. Which makes sense, you develop muscle by breaking down the muscle filaments through exertion and your body increases muscle size during the repair process.
Ralph by all accounts is a very inactive person. Just think about what his daily routines are likely to entail - he sits his fat ass down on his chair and then fucks about on his computer until he fires up his stream and does much the same. During this period he will eat, shit and drink. Do you really think Ralph goes out shopping? No, of course he gets it delivered! A walk? No, he... May can drive him around.
This fat fuck referred to mowing the lawn as a workout. His lawn was tiny, it was an electric lawn mower - not one of those old manual ones that did require someone to heave them about.
Ralph has no muscle because he doesn't use them. He doesn't, even during coitus Ralph doesn't do anything. Just fat on a withered, Mr. Burns-esque body.