I would have questions related to discussion under this article. Bruce Schneier answered questions regarding key size vs number or rounds:
Why do you need more rounds with longer keys? And how did you come up with these seemingly arbitrary numbers for more rounds?
When block ciphers get broken, they're invariably broken due to diffusion failures. What more rounds give you is more diffusions ... the key is twice as long, so it takes more rounds to get complete diffusion of the key.
As to your second question, choosing the number of rounds for a cipher is a combination of experience and guesswork.
For example Threefish 512 has 72 rounds and Threefish 1024 has 80 rounds.
I would like to know your opinion on theses questions:
If Threefish 1024 would have only 72 rounds (as Threefish 512), could it be in some way less secure than Threefish 512?
If the number of rounds is a "guesswork", could Threefish 1024 (full 80 rounds) be still less secure than Threefish 512 (72 rounds), because of "incorrect guesswork" (8 additional rounds wouldn't be enough to provide more diffusion to maintain the longer key)?