I have ripped the album Damnation by Opeth using dbpoweramp (normally I use EAC, but I was on another computer) and with the quality 7. The total size of the album came to be about 56MB. When I was looking on allofmp3.com I used their Online Encoding Exclusive service to encode the same album at the same quality level and it said it would be about 66MB.

What possible variables could result in such a large file size difference?

Different encoder versions?

Yeah and possibly allofmp3 is full of music from shit sources. You'll get what you pay for.

That too

I think allofmp3 calculates the expected download size from experienced avreage bitrates - on quality-based vbr this can get totally off-target, depending on the complexity of the music. You will not actually know how many MBs you will get until they are downloaded.

Check out the different lossless formats - they are all calculated as 900 kbps, even though 'everyone' knows there will be differences due to music complexity and encoders.