They've been the only North American carrier reducing capacity from Ireland in recent years. ATL went from year round L1011 to MD11 to B772 to A333 to a seasonal B763, and JFK was 10 x weekly B763 peak season until a couple of years ago. 4 x daily AA and 4 x daily UA must have hit them hard?
Indeed they are the only carrier to not increase capacity but contract slightly but that has started to chnge recently.
JFK overall capacity during winter will has increased during winter 14 and now winter 15, granted stopping ATL for full season in winter 14 helped that and increased capacity to JFK.
The 10 weekly peak season JFK only operated for one season (? - @most 2) and I think we may see the A332 or 3 stay around for next summer to JFK over ATL. Summer capacity to JFK is up this year as DL previously operated B767 peak summer and winter and B752 outside of this in the last year or two in summer.
ATL is heavily dependent on transit numbers and as capacity increases and new routes have happened they have squeezed DL on that front.