The two commentaries I've tried are Yellow and Green (Book 8 by A.M Bowie) and those by Geoffrey Steadman mainly book 1 http://geoffreysteadman.com/files-herodotus/.
Bowie's commentary takes up something like twice the space of the text itself. However it includes a lot of historical interpretation stuff that would be helpful if I was reading a translation but for me at the moment when reading the Greek is a struggle much of the comment is just a distraction. While there were bits that were helpful to me in getting to grips with the text they were rather sparse.
Steadman devotes half to the text and half to the commentary plus vocabulary. Hence the commentary has only half the space as the text. However, he is entirely focused on providing help to those like me who badly need help with the grammar. His comments are briefer and so he packs more in (though perhaps on occasion a little too brief). Steadman for me wins hands down.
As someone struggling to read Heroditos I have only been able to sample small parts of both books so my impression is not representative of these commentaries as a whole.
Anyone tried Bryn Mawr one? http://www.hackettpublishing.com/bryn-mawr-commentaries/bryn-mawr-commentaries-greek/herodotus-book-1