Sharply focused files will be larger than out of focus files. Thats because there is more information in a sharply focused file.
Make sure your lens is accurately focused, sometimes the AFMA must be adjusted differently for each body. The Camera body produces sharp images, if they are not, its a focus issue or a operator error. You can check focus by placing the camera on a tripod and taking a image, then using live view perform a live focus(AF-ON Button until the ring around the focus point turns green) and then capture the same image. Live focus normally provides the best possible focus, so if it is noticibly sharper, you need AFMA to fine tune autofocus with your lens.
In camera jpegs may have a lot of NR and appear to be soft, Shoot with raw and use the advance Release Candidate versions of Lightroom, or Adobe dng converter. A new DPP Version is expected this week, and we may see some camera firmware improvements for jpeg sharpness before long.