The size of the derivative is counted for storage, not the original. For example, if you upload a TIF and CONTENTdm converts it to a JPG, the compressed size of the JPG is counted into the storage allotment, not the TIFF.
Make sure you pick the appropriate setting in Project Settings Manager > Images & Thumbnails. We also strongly recommend using JEPG2000 instead of JPG and pick Create display image during the step-by-step upload.