CompuServe Messages

Hi, We just ran out of space and decided to give JPEG a try on a long animation run. Funny thing is that about 1/3 of the files are about 1/4 the size of the rest: 50k vs 200k. We are rendering the project on four machines, and I haven't looked to see if there is one machine in particular that is packing them smaller, but I have do wonder why this might be happening? Thanks, Donald