Flavour in different cuts

I read somewhere that different cuts of beef have different flavour intensities. The general trend was that the more the muscle was used (ie: rump), the more flavour it had but the tougher it was and the less the muscle was used (ie: fillet), the more tender but less flavour it had.

So it seems like flavour and tender are inversely propotional to each other.

Is there a basis behind this? Whilst we are at it, what is it that makes a cut 'tougher' than other cuts? Is it that the meat is more 'dense'?