If it's a tube train, then it doesn't matter that it has lots of carriages, because the carriages are likely connected to each other with a bendy springy section. Being articulated like this gives them a wider turning circle.
This is how the trains on the London Underground are like, for example.
It's how an articulated bus for example, can get around sharp-ish corners, despite being very long.

As for the original question, the tubemap is just a simplification of the routes. In order to make it easier to understand, as well as capable of being squeezed into a smaller space. The turns they make on the map only vaguely relate to the actual turns the tracks make.