The material a frame is made from is not very important. Most important is a good fit, which includes both the height of the frame, and the length of the frame. Spend some time at the bike shops in your neighborhood looking at their bikes, and getting fitting suggestions. If you end up buying a used bike, have your neighborhood bike shop tune it up, and buy a good lock and good front and rear lights to help compensate the shop for the time the staff invested in helping you get a proper fit.
Bike frames produced in a factory have to be a certain strength to cope with whoever rides it so lightweight riders end up with over-built bikes. Custom bikes can be made from much lighter grades of material (steel, Al or carbon) but the lightest production bikes are hard to beat in terms of weight.
If you look at the weight-weenies website you can see the frames that people start with to built ultra-light.
Weight is only a big consideration if you ride a lot of steep hills or big mountains. On the flat, aerodynamics play a greater role in going fast and over long distances, comfort is the primary factor. It is however, always nicer for us skinny riders to be matched to skinny bikes.