I wouldn't recompress anybody's stuff without first letting them know, and getting their OK. Even if I was to decide to make it policy that I would go ahead and do it without an explicit OK for each instance (not currently on the table), then I would at least make a prominent posting letting everyone know that before hand.
Storage is definitely one consideration. If it's a video that has a lot of motion or detail in it, or is of sufficient length to require a huge file size to maintain the quality, then so be it. But let's keep in mind that there are a good number of amateurs here who don't know squat about the quality settings when rendering their material. Some videos come through here where the camera was on a tripod, and there is very little motion from the subject, just in the center of the frame. But the producer just ticked the highest quality rendering setting, and we have this huge video that could easily be compressed a lot more, with little or virtually no quality degradation. These are the videos that I am targeting, not the ones that are very professionally produced and obviously need the filesize to store quality image information. Even then I would talk it over with the producer. Remember I am a producer as well and I'm fully aware of the blood, sweat, and tears that goes into quality control, from initial model contact to the final render and upload. It's not my goal to be greedy an re-render all your shit until it's nothing but jpeg artifact blocks!
The other consideration is making file sizes smaller for user convenience when they download. We're in a broadband world now, but some users believe it or not do have trouble downloading these 1GB files.
Finally, I do not charge for storage or bandwidth. There is no consequence or deterrent for people to upload files of any size and just leave them here...forever, whether they sell or not. I'm not aware of too many small sites like this one that do that. I can't afford to simply not think of how to reign in on things if they do start to get out of control.
Stay messy, my friends.