oh, hmm. This may be something I didn't realize needs to be updated.
A point of confusion is that WordPress (and Imsanity) look at the images files to get the size, width, height, etc. But actually that information is only looked at once when you upload the file and then saved in the wp_meta table. From that point on all file size information you see is from the data in that table - not from the actual file sizes. So there is always a possibility of that data getting out of sync with the actual file in which case WordPress metadata says a file is 5Mb but the file itself is actually only 2Mb. Generally this is just informational anyway and doesn't affect anything, but in the case of a site with quota limitations it might make the total size incorrect.
When Imsanity does it's bulk resize, it has to also update the wp_meta table to match with whatever the new width and height is. As it turns out, I'm not 100% sure during the bulk resize that I am also updating the metadata indicating the disk size of the file. I will have to take a look at the code to refresh my memory.
Also it would make sense that the total quota percentage may be something that is calculated separately, because it would be expensive to calculate that every time you look at the dashboard.
One thing you might try - just upload a new image and see if the total quota updates. If it's something that WordPress calculates only when a new image is added or deleted then that might be enough to correct it.
Thanks for the report, I'll have to do some research on this one...