2 things to remember about:
The 60D is 18MP, the XSi is 12MP. Your lens captures a certain amount of details. When you use the 18MP sensor, all the details spread across more pixels. When you compare the 12MP and 18MP photos on a monitor, the 18MP one has bigger dimensions, which with the same amount of detail should result in a bit softer photo. So, in theory, when you resize your 18MP photo to 12MP and compare it to the photo from XSi, it should be of about the same sharpness. But that's just theory. In reality, the 18MP sensor will capture a little bit more details. However, if you view both pictures at 100%, the 18MP one has to be a little softer compared to the 12MP one.
Also, remember that the Rebel is an entry level camera. Camera manufacturers are known to be messing with RAW shots on these, so they are not exactly 100% RAW. Images are usually sharpened in camera. Often also the anti-aliasing filter is being weakened. Something to remember about when comparing images from the Rebel and a 60D.