So this is how I see it, @steemit holds the largest stake which means he has the largest bandwidth

Sort of. Part of the purpose of the reserve ratio is to account for unused and underused stake like the steemit account (and in practice most other large accounts). Since those accounts are not using their full share of bandwidth, the reserve ratio grows (or stays) higher, and that unused portion becomes available to every else via a (temporarily) increased allowance.

That is why also @steemit is undelegating his sp to newbies before but what about those who havent got enough sp?

As I understand it, the account is supposed to be undelegating from accounts which have been misbehaving (in this case many accounts controlled by the same person/group and mostly spam self voting). To the extent those accounts are cut off from the delegation and more limited in bandwidth, more bandwidth should then be left for everyone else.