From what I have read, weight gain due to insulin resistance is a sign that the body is producing plenty of insulin - which also ramps up the insulin resistance. Which ramps up the hunger, and the blood glucose levels, but the glucose isn't reaching the muscles or the organs which are insulin resistant - but fat cells don't get as insulin resistant, so the glucose is carried into fat cells by the abundant insulin. Jason Fung's blog
Intensive Dietary Management is a great source for this info, and on how insulin works.
By contrast, sudden weight loss is (may be) due to insufficient insulin, uncontrolled high blood glucose, glucose being peed out via the kidneys, and the body effectively consuming itself to try and feed the cells which are not getting the glucose due to insufficient insulin.
The first situation is common in T2s, while the second situation is common in untreated T1s.
However, if T2s are untreated for so long that their beta cells die off (because beta cells die in the presence of too high glucose, and dead beta cells can't produce insulin) then the person can lose the ability to generate insulin, their insulin levels fall, their insulin resistance falls too, and they start to experience insufficient insulin (and weight loss).
There is a figure that is often bandied about (I am sorry, I cannot quote a specific source) that says most T2s have lost half their beta cells by the time they are diagnosed. If this is correct, it must mean that the remaining cells are working hard, and are more likely to fail, especially if blood glucose continues uncontrolled. If that were to happen, then the T2 would (presumably) transition from insulin resistant and fat storing to insulin deficient and losing weight. I believe that this process is what leads T2s to start injecting insulin.
Obviously, it is much more complicated than that! But
@Mep and
@leslie10152 and
@Kristin251 may have first hand experience of this.