File type for CNC

Hello, I am using Visual Mill 5.0. It takes only STL, IGES and Rhino
files. I tried IGES first to machine a cylindrical channel of .040"
OD. This was done on a HAAS with a step over of .001" and a .015"
ball mill. The end result was instead of producing a circular shape.
The cross section looked more like a v nothch shape. I check the tool
path. The tool path actully resembled a V shape as well instead of
being fully circular. Then I tried to reprogram using STL file using
a step over of .005". The tool path was much better and it was
circular. The catch is that the CNC file became very large. On this
small part, the STL file needs 50,000 lines and will take 9000 minites
to finish. My question is, what is the difference between STL and
IGEs and how does a CNC software handle them? Thanks

Tony,
North CA