First past the post tends to result in big government

In any political system the people and parties in power are able to do what they want subject to the constraint of being democratic. This means that (because they are the government) they are able to dictate the law. The law is defined by the government, which means that they are able to appropriate wealth and property if they wish. Elections enable the people to force the government to release money and power back to the people, so if the voting system is inefficient then this process will happen to a lesser degree and the people will be poorer and the government will be richer.

Part of the problem with the first-past-the-post voting system is that it tends to favour just two political parties which means that, in the end, they just compete against each other and not to serve the people. If more parties can win then each party will feel they are being 'attacked' from different directions, but with a two-party system they must worry only about what the opposition is offering to the people. So if there is less competition for the political parties in an election this is bad for democracy and will result in more poverty for the people as the state appropriates power (relatively) unchecked. To mitigate against this process it is possible to ensure that smaller, minority parties have a better chance and this means altering the system to get something close to proportional representation.

First-past-the-post tends to result in big government whereas proportional representation puts more pressure on the political parties which means a smaller state and more prosperity for the voting public.