Both my iMac and iPhone are on the same wireless router (Airport Extreme).
But my iMac is pulling 50Mbps down while my iPhone 4 is maxing out around 20Mbps.
What could be the reason for that?
The processor and hardware architecture of the iPhone (and mobile devices in general) cannot handle the processing required to download data nearly as fast as your iMac. This combined with a million other variables, including the fact that the antenna in the iPhone is much smaller and could receive a weaker signal even though it is the same distance from the transmitter as the iMac, are among many reasons the transfer speeds are slower on your iPhone.
In short, there is a processing bottleneck somewhere in the architecture of the iPhone that is not present in the iMac for transferring that amount of data.
I reckon it is because you need to squeeze the receiver in a much smaller space. Because the receiver is smaller, you have to limit what you implement. It's probably the best they can do for that size, without making it to expensive.