Sherlock Season 4: 20 Easter Eggs & References In 'The Final Problem'
15. Young Mycroft
In the flashbacks we see of the Holmes children's childhood, we see that Mycroft was a somewhat heavily-built child. This is both a nod to the original stories and the culmination of lots of references in the show itself.
When Mycroft first appears in Doyle's The Greek Interpreter, he is described as "massive." In Sherlock, the detective has previously needled Mycroft on his weight loss and we have seen Mycroft maintaining an exercise regime to battle his bulging belly. In The Abominable Bride, Sherlock's Victorian vision of Mycroft was an enormous man constantly surrounded by food.
Though it has not been seen clearly on screen, part of the set decoration in Sherlock's bedroom since the beginning of the show is a photo of young Sherlock and a young heavily-built Mycroft. That image has now been brought to life.