Noob... UNO maxed out but Mega wont run Matrix?

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On a MEGA it is possible that the table is outside the first 64k of FLASH/PROGMEM. If that is the case you have to use 'far' (32-bit) addresses:
uint16_t j = pgm_read_word_far(&XYTable[i]);
Try that and see if it fixes your pattern.