I'd also like a way to turn it off. I think it's great to have on large, multi-megabyte images. But more often than not I've found having to click on "View Original Image" to be a pain.
What I'd most like to see, if it were possible, is to allow users to set their own thresholds. Obviously, the server would set the lower limit (I guess 1000x1000 and 512KB right now?) Users could then raise the thresholds. For example, a user might set her thresholds to 2000x3000 and 1MB. In that case, the resampled images would only display if the original image was larger than 2000 horizontal or 3000 vertical or 1MB.
If bandwidth is a concern, the amount the user could raise the thresholds could be capped. I'm not sure what a suitable cap would be. In the most extreme scenario, a user would always choose to view the original image, thus using up the bandwidth of the original image and the resized image.
When imposing any limit, I think you have to consider how many images will be "stupidly affected", resized for little to negative benefit. For example, the current hard limit is 1000x1000. If a large number/proportion of images fall between 1001 and, say, 1200 in some dimension, there are going to be a large number of images "stupidly affected" by the limit. It would be interesting to do an analysis of the current database, considering image dimensions and limits. Perhaps you could even factor in the popularity of an image, giving it more weight in the analysis if it has a higher view rate.
I think leeway is superior to a hard limit.
For example, if images over 1000x1000 are resized to 800x800 (so 801x801 images are left alone), then a source image that's 900x900 ends up getting to be bigger than the 1000x1000 image, which is even weirder.
That isn't an issue at all. As StarlitVoyager already said, it's supposed to do that. Resampling an image for little gain just wastes harddrive space and could hurt bandwidth (depending on how aggressively you compress the resamples). For example, if an original image is 1050x1050 and 200KB, perhaps the resampled image is 1000x1000 and 180KB. In this case, if more than 10% of people choose to view the original image, you've actually hurt bandwidth.
Another hilarious example is post #209733. The resampled image is actually LARGER than the original, 244KB to 145KB. A sanity check should be introduced. I've opened a ticket #209.