You can rebuild medium and thumbnails through the program and any lowered size or quality setting would make them smaller.
As far as your main images they are never touched except when you actually upload them and sized accordingly to what size thresholds you have set. I use 1500 as a max for height width.
If your trying to rebuild all your original images you would need to create some type of script to do that. Not really some easy one or two lines I could post to help point you in some direction.