@sigmanutz Ideally you'd look at the dimension of the 64Gb drive with df and note the number of total blocks of the / partition, then compare to the df output from your 128Gb card (also in blocks) and note the number of blocks used.
But, just to make sure, just backup the biggest ROMs folder to your PC and you're sure to get a smaller image and the creation process will also be shorter. You can then move back the ROMs to the new system.