Basically there're two factors. As voltage gets higher the current gets lower and losses get lower and that allows for thinner wires. On the other hand as voltage gets higher better insulation is required everywhere - posts have to be higher (so that no discharge into the ground happens), distance between the wires needs to be greater, and much better insulation is needed in the transformers at the ends of the line. So raising the voltage reduces transmission losses and wires cross-section, but induces lots of problems with high voltage itself. That's why the actual used voltage is a tradeoff - high enough to not lose too much energy as heat and not too high so that the system can be manufactured and run.