For Medicham, it technically doubles the attack stat based on the attack stat itself, not its base stat. To put it in perspective, Azumarill (who has Huge Power which is the same thing) when you determine it's fully invested in Attack and an Adamant nature has about a base 149 Attack stat. That would be its stat if you strip down from its fully invested, then doubled attack stat. Basically, iirc, Medicham's true attack stat with full investment (idk about adamant) has around base 170 attack. So it's all about the stat and nature and not really about the base stat itself (base 100 for Azumarill is not very powerful).
But I agree that Medicham might get an evo, but one with only a tiny bit more attack, and mostly bulk and speed added. That would be so cool to use (but extremely over powered probably).