Women in Jannah vs Jahannam - who will be the majority in either one?
Tl;dr:While initially women will indeed be the majority in Jahannam for some time, they will eventually end up being the majority of the population of Paradise.
The original hadith about women being the majority of the population of Jahannam is authentic and valid - the hadith itself stands as a warning to refrain from certain specific behaviours (cursing and ungratefulness to husbands).
It does *not* mean that women will *always* be the majority in Jahannam.
There are other ahadith, and specifically a statement/narration from Abu Hurayrah (ra), which mention that there will be two human wives per every Muslim man in Jannah - thus, literal two-to-one population.
This is the opinion of Abu Hurairah (ra), Ibn Hajar, Qadhi 'Iyadh, and Ibn Taymiyyah.
There is another narration wherein it is said that a minority of the population of Jannah will be women. This has two explanations: either that the narrator narrated the meaning that he extrapolated from the hadith about women being the majority of Jahannam (i.e. if they are the majority in one place, they will be the minority in the other); or, that this specifically refers to the time before which the believers have been released from their temporary period of punishment in Jahannam. After that, they will be purified and freed from that punishment, and they will enter Jannah.
http://www.islamweb.net/fatwa/index.php?page=showfatwa&Option=FatwaId&Id=67212
http://majles.alukah.net/t7457/