Progress bar should be based on size, rather than file count

There's like, 3 pixels left in this bar and it accounts for about 2/3rds of the data...

This is a great idea but I think it should be based on user preference :)

Not sure I see much rationale for that. Why might a user prefer it by file count, when different files take wildly different amounts of time to process?

I suppose compresstimation throws a spanner in the works a bit, because a large file might take quarter of a second or three minutes to process :/

Why might a user prefer it by file count

I don't know. Users are weird. They may be used to seeing compression progress bars one way or the other. Also, it might take longer to compress a huge number of tiny files when there are more numbers involved. Some people don't care about how fast the progress bar moves, they just want it to finish faster.