Compression remains the same. A better (but not perfect) definition is that there is an ideal combustion pressure to force the piston down. Iron heads retain the heat, so say this ideal is reached at 9.3:1 compression on my theoretical example. Any less and you loose power, any more and you detonate. The aluminum heads bleed much more heat out of the chamber, so another full point of compression is required to maintain the same amount of heat in the combustion process. So the process sees the same amount of heat with 9.3 iron heads and 10.3 in aluminum heads. Over simplified, but it gets us close to what is going on.
There are also limitations in stock cast iron heads that limit the amount of porting that can be accomplished. Since the aluminum heads are designed from the beginning as a performance head, there is more meat designed in around the port area so a better port can be designed, and better flow is the result. Aluminum heads save weight, and they weigh half of an iron head. Aluminum heads are cheaper and easier to produce than iron, and are also much easier to machine and port. Because of this more companies make aluminum heads - and that competition brings a better product to the market, and keeps the prices down for us.