OLEDs use considerably more power (the power varies according to color) and typically have a smaller pitch - so to get the same physical size screen would require a higher resolution. Or, to put it another way, using the same resolution with OLEDs would result in a screen that was physically smaller. I believe OLEDs are more expensive than TFT but I'm not sure how the price compares to the ICP variety. Also, the useful life of OLEDs is currently less than that of TFT LCDs.