Is there any truth in women saying 'my bum looks big in this'?

Yes, there is a scientific reason why wearing some colours will affect the apparent size of your bum. The reason that we look larger in pale colours, and thinner in dark (especially black) ones, is because our perception of the shape of the surface depends upon minute shadows and patterns on the surface. Compare how easy it is to spot a wrinkle in a light coloured shirt compared with a dark one. So if you wear dark trousers you cannot see any shadows, because the background is dark, making it more difficult to perceive the actual shape of the bump underneath. Patterns also play a role. If you wear something with vertical lines that start off parallel, then move apart, then back together again, to form a fish-eye shape, looking at the pattern gives the impression that there is a bump underneath, even though there isn't.