The factory farms in which the vast majority of British chicken is reared have limits on how many birds can be crammed into their hangar-like sheds. Space is everything and with it being so limited for chickens they aren't growing as well as they should and are therefore being slaughtered sooner. This means us as customers in our local supermarkets aren't getting full value and will see that chickens aren't as big as what they used to be.
The solution to this is to purchase organic, grass fed chickens that are looked after better and aren't crammed into very confined spaces. You get what you pay for and although organic is more expensive, at least you know you will be consuming a more nutritious chicken!
Reference - Daily Mail - http://www.dailymail.co.uk/news/article-2585833/Revealed-How-supermarket-chicken-shrunk-millions-birds-suffering-worse-conditions.html