Please enlighten me on this...

I noticed this on another engine I worked on, but I never questioned it. But why is the intake valve larger than the exhaust valve? I was thinking about getting 8 more intake valves and having a machine shop enlarge the valve seats on the exhaust side to accomodate the larger valves. Is this a bad idea? Also, are the stock intake and exhaust valves made of the same material? I am just trying to get as much flow from an LS head as I can.
Shaun Ladd