malc4d
You have hit the nail smack bang on the head.
Take a healthy normal male with say a 38" chest and measure the dimension across upper arms and you will get at least 23".
The width of economy seats are usually 16", 17" or occasionally 18". Add 2" each side for arm rests which gives an available space 20", 21" or 22".
Any 'average' male allocated a centre seat is bound to occupy both arm rests ad possibly more. This is not something new, it has been a fact for decades.
I firmly believe the lack of shoulder space is the main reason why economy travel can be so uncomfortable. But hey, when has economy passenger comfort ever been a consideration with any airline.