Whose to say this wasn't his idea in the first place? Nobody is for sure what went on in the private Apple meetings. It's been cited that Apple usually plans for products up to 5 years in advance — and he clearly was involved with the iPhone 5 and iPad Mini production (released less than a year after his death), so allowing different sizes that went against his original standards weren't too unlike him.
Anyway, even if, this is Tim's company and Steve told him to run it how he saw fit, and the market demanded larger phones. There's really no reason to think that Apple would never offer multiple sized phones when they are becoming more important and more efficient as computers (where they offer 13" and 15" models).
We haven't even really entered the "post-Jobs" era anyway, both the Watch and impending TV have been reported to being in the planning back when he was still running the company. What comes after those might be the real telling of what Apple does without Steve.
But anyway, Apple needed to do this. Both models seem to be selling great.