I think the dose is just different for every cat. Probably cats with some pancreatic function need less and those without pancreatic function need more. Insulin resistance plays a part and seems to be somewhat reversible in cats. Raise the dose high enough to achieve lower numbers and you can "break through" that resistance. Then, as with Tigger, the dose can be lowered. Of course, non of that would apply to an acrocat.
Weight does play some role in it, at least in humans and probably in cats. When I got Jeddie he weighed about 8.75 lbs and was 1 month past diagnosis. I suspect he was diabetic a long time before anyone noticed. He was put on 1u of vetsulin and DM food and not retested at all (!!!) for a month. I had him off insulin after just one .5u dose of vetsulin. He ate like a pig because he was so thin and had been staving and started gaining the weight back. He is part Maine Coon and had weighed 13 lbs. After 3 months he had to go back on insulin, I think he was about 10 lbs then. His weight seems stable now at 12.5 lbs and he is still on insulin. So, it is what it is.
Bottom line is that YOU have to do what YOUR cat needs. With Mickey that seems to be systematically raising, letting it settle, etc.