I can look at it from both sides of the argument because I've been both anorexic and slightly overweight and everywhere in between. Topshop are simply selling to their demographic which is I would imagine women aged 16-25 and between a size 6-14. They shouldn't be forced to sell larger sizes just because some people need it, but my idea would be that they could have a smaller number of items online in slightly smaller sizes and slightly larger sizes to cater for that smaller market. At the end of the day we all need clothes no matter what size we are.
In an ideal world all shops would stock clothes in the average national women's size and all those sizes a standard deviation either side of that and other sizes could be sold in specialist shops, but this isn't how shops work. But if you take into account the fact that many more people in the UK are overweight than compared to those suffering eating disorders that size is likely to be skewed towards being slightly more than desired.
Topshop I would say is most geared towards normal to skinny women, even though I'm a neither overweight nor underweight anymore I find lots of their clothes don't look great on me but that's because I'm quite short and have a curvy figure - but do I worry about it, no - because there's lots of other shops I can shop in.