But no one is arguing that being overweight or obese CAUSES diabetes. It does significantly increase the RISK of diabetes, although there are obviously other risk factors too.
Lots of people argue that being overweight (or overeating, or eating too much sugar) causes diabetes. I think it's a form of victim-blaming or fat-shaming.
But even if we're talking about risk, we could just as easily suggest that being overweight might be an indication that someone already has diabetes. In other words, being diabetic might increase the risk of being overweight. It's just as reasonable a conclusion from the evidence we have. Weight gain is usually observed before symptoms of diabetes, but we still don't know which comes first; we only know what we observe first.
Anecdotally, I can remember sitting in a diet-for-type-2-diabetes clinic in which my DH was the only person there with diabetes who was not overweight. It is hard not to conclude that there is some link.
Agreed. Confirmation bias is a powerful thing, especially when the evidence is anecdotal.
The next step is to ask what kind of link (if any) there might be. So you have observed that many people with diabetes are overweight. Your husband is not overweight. Therefore some people with diabetes are not overweight, but the majority are. Meanwhile, the vast majority of overweight people are not diabetic (which you don't initially observe, because those people are not at the diabetes clinic, but you can look up the statistics). If you think there may be a causal link, you might reasonably conclude that diabetes often causes weight gain, rather than the other way around.
For the record, I'd be perfectly happy to accept that weight gain causes diabetes, if anyone ever comes up with evidence that would satisfy the Cochrane reviewers... Until then, I think it's important to challenge the assumptions that seem to inform some of the fat-shaming I've seen...