Most of the microSIM trend is based on the design factor. A smaller microSIM tray means more space for a bit larger battery. Battery life (thus size) is a main driver of design right now as 3G and LTE can sap battery life. Millimeters count especially when coupled with the desire (no not THAT Desire) to make/keep the phones thin.
Despite Steve Jobs belief that the iPhone was/is the right size at 3.5 inches because that's about the average reach of using your thumb on it, I'm betting that the next iPhone will be a bit larger. Not to keep up with the Joneses (HTC, Samsung, etc., but because the extra space can allow them to implant a larger battery thus being able to brag about better battery life in a 3G-LTE World.