Actually 16 man hm in the current tier is way easier then 8 man (assuming you have groups of equal level), there's not a single fight were you would need more then 8 dps, so you could go with 6 healers for all the hm content without problems, for example.
The only fight i would consider a little bit more difficult in 16 man is Dread Masters and that's mainly because the optimal tactic for the first phase of the boss depends a lot on the group setup and you need a slightly better preparation before entering the final phase. The rest is easier.
8 Man guilds usually have problems with 16 man because of
a) a lot of lag, as they might no have upgraded their pc for 16 man Operation (there's quite a big Performance difference between 8 and 16 man)
b) not having equally good people or not even enough people to get a full group
Draxxus, Corruptor Zero, Brontes, Bestia, Tyrans and Raptus all have mechanics that can be cheesed away on 16 man. For Nefra and Grob'Thok you could theoretically take more then 8 healers and still wouldnt have problems with enrage.
Of course group communication and coordination is more difficult when you have double the people in the raid. But in terms of actual requirements for each individual person, 8 man was always more difficult (atleast as long as i remember) then 16 man.
No one is going to run 8 DPS on 16 man HM DF/DP unless they are already geared out with mostly 78s, which already defeats the purpose of claiming any sort of difficulty, regardless of whether it's 8 man or 16. Hence, my point about people overgearing 16 man (as well as being more experienced) and then trying to say it's easier is ridiculous.
As for mechanic cheesing, please explain exactly what mechanics you can cheese on those fights on 16 man that you can't on 8. Does your team magically live when standing in the laser at the end of Corrupter Zero if you have more healers on 16 man? Or if people stand in the Inferno repeatedly on 16 man? Or if they stand in the red circle on Bestia? No, they won't. It's irrelevant how many healers you have if your tanks don't understand that they have to re-taunt Draxus when he does his aggro drop or if Raptus cleaves your entire team constantly or knocks your tanks to their death. There are plenty of one-shot mechanics in HM DF/DP if you fail to do them correctly so I don't know what you are talking about when it comes to cheesing mechanics.
If you are talking about being able to have more healers, then you can do the same on 8 man as Nefra and Tyrans can definitely be done with 3 DPS and I'm sure there are other fights as well.
Beyond group communication and coordination, positioning and survivability are far more important and skillful on every single player in 16 man. For example, on 16 man HM Brontes, healers can be killed instantly once the Fingers spawn in the beginning of the fight if they happen to target the same person. Also, the massive spikes you see only on 16 man are far more dangerous than those on 8 man, which is why it's a joke when 8 man teams try to complain about spikiness. Positioning is crucial in many fights especially since the areas are designed for 8 people and you have to fit in 16 people (Bestia is a good example).
In the previous tier, 16 man NiM TFB and SnV was far more difficult and I'm fairly sure it's well-known that NiM EC in 16 man was much, much harder than 8 man. So maybe 8 man was harder in the days of EV/KP or HM TFB and SnV, but I have no idea since I almost never did 16 man then. The only fights in general that are much easier in 16 man are SM ones and world boss fights like Toborro's Courtyard and the one in the Gree event.