Thread:Project:Support desk/My mediawiki thumbnail is weird

After I upload an image to my wiki, the thumbnail turns out to be the same file size as the image I uploaded. For example, I upload image A and A's origin size is "1,024×768". The code becomes "". The file size is still the same as A.jpg. In Wikipedia, it seems to be another solution to thumbnail. Take A.jpg for example again. A's origin size is "1,024×768". But the thumbnail code is "". It means it automatically creates a smaller picture against the origin one. How could my wiki do the same thing like that?