When did two become four?

Would anyone care to tell me when Barney suddenly grew up and become as big as Daisy?  Is it camera trickery?  He's meant to be 2 years younger?  
Ahh yeah, I should also add this isn't an exceptional photo (unless they are YOUR kids).  
Why, well picture this..... as a parent with two children you usually spend all day repeating the following phrase: "give in back to (insert name as appropriate)" which becomes pretty tiresome.  So to see them sat next to each other (kinda cuddling) with no bickering is just DREAM.......and a little bit down to the hypnotic powers of television.