The post on relative relatives went down quite well, but hindsight is a wonderful thing and it seems now to me that I really should have made it more general. The thrust is of course one of context and while pretty much everyone has a grasp of this, it does seem to slip past some.
In short, something can be large, small, fast, slow or whatever in context, but no overall, or in a wider context. Compared to other spiders or insects, bird-eating spiders are big. However, this is contextual – compared to terrestrial vertebrates, they are pretty small. Thus, they can be described as both large *and* small. There is no contradiction, but there must be clarification. Small compared to vertebrates, big compared to spiders and insects. Some snails are much faster than others, but none are fast in the grand scheme of things.
Remember then to use context when using adjectives, and remember to check the context when reading them. It’s perfectly possible and accurate for something to be big and small and fast and slow and young and old and long and short all at the same time.