The appearance of the Mashiach depends on the appearance of Israel. If Israel merits the Mashiach because of the righteous of the generation, then the Mashiach who appears is in the form of a righteous man himself. If, however, Israel merits the Mashiach because the time has arrived, then the Mashiach appears as a sickly person. The Ramchal says that fundamentally every gadol ha'dor from the beginning of time has the a certain power of the Shechina, but the Mashiach will have the Shechina dwell upon them like it did in the Temple.