In a country of, say, 300,000,000 people, it is easy to bribe 150 politicians. Having a total legislature of 150 people in a country of that size would be quite easy to manipulate, and constituencies would be huge, meaning politicians would have no real ability to connect to their constituents.
Large countries are usually federations for a good reason.
Whatever you do, the needs of a functioning legislature and the needs of connecting to your constituents are orthogonal. They may be made to fit each other in special cases, and it's great when it's the case, but if the size of the country makes such alignment impossible then it just is and you'll have to choose.
