The insulin resistance element is of course worthy of consideration. But one other element is that higher doses of insulin might actually provide more muscle growth, independantly of nutrients uptake/storage by the muscles. The reason is that insulin (especially lantus) can greatly increase igf-1 levels, and it can do so by itself (without the concurrent use of GH).
That is actually a big debate in diabetes research: whether lantus is safe to use long term in diabetics because the igf-1 increase might increase the risk of cancer.
It is also possible that high doses of insulin might bind to the igf-1 receptors.
Because of the health risks and potential fat gain I do not recommend it, but its the main reason you will see pros use such big insulin doses, not only because huge GH doses and the constant use of insulin can increase insulin resistance (although it DOES also play a role).