Wow.
Really didn't think "entering the large screen market" (I didn't even know that was a market) would have such an effect.
So bigger really is better?
Samsung was one of the main reasons Android was thriving for the past ~4 years. Their phones were a solid combination of price and quality. Very good all-round phones, and thus serious competition for the iPhone. That was quite an achievement because Android wasn't really easy to work with as a manufacturer, so to be able to make good all-round phones in all segments was the main reason of their success. Despite this, Samsung always had their weak points. Uninspiring design, bloated firmware, questionable continued use of plastic, you name it.
Fast forward from 2010 to 2014, and the situation is different: stock Android is a really good foundation for a phone. Android has become less resource hungry (or the resources have gotten better faster than Android has gotten more resource hungry, whatever), and cheap Android phones are "good enough". The value proposition of Samsung has basicly vanished: not because their phones got worse, but because the competition (mainly due to stock Android) got better. Also, the competition is often cheaper.
What remains is a brand that relies on their name from the past (which is actually only the past few years, but is an eternity in today's smartphone world), offers hardly and advantages over the competition anymore but still has the disadvantages.
Two to three years ago, there were plenty of reasons to buy a Samsung. Today, not anymore. Hence the tumbling profits. I don't see Samsung becoming the next Nokia or something, they will still remain relevant, but their sole reign in the Android kingdom is gone and that era will not return unless Samsung is radically going to change their smartphone strategy.