The .G64 file format can be compared to a D64 in the same way that TAP files compare to T64s. It stores extra information found on 'real' 1541 disks that aren't present in D64s. Like TAPs, this bloats the image size somewhat, but the benefit is that you have a better representation of the true media (and you might be able to run special fast loaders on them as well as some forms of copy protection, if you have the means to copy the disks into that format).