I am using C#for Arcobjects.
To change the width of a line, use the ISimpleLineSymbol.Width property. Please view the following code snippet:
The ISimpleLineSymbol interface is implemented by ISymbol. You will need to cast ISimpleLineSymbol to ISymbol, and then set the ISimpleRenderer.Symbol property to ISymbol.
Please view the detailed code sample below to see the implementation of a renderer.