Because the software you are using has it's own notion of what should be in the STEP file. It might just be that the software is padding real numbers out to extra digits that the original software did not do. Or it might be taking one format for surfaces and edges and having converted them to the internal format, writes them out as something completely different.
I could give a square as (0,0), (1,1) as opposite corners or (0.0000000000000, 0.0000000000000), (1.0000000000000, 1.0000000000000) for a huge change in file size with no change in information.
In short, STEP files are not unique when describing the geometry.
It may also be that the software has added default color information for each element. Or some other factor.
It's easier to see if you can get the original STEP file maker to send a file of a cube, which you then read in and then create a new STEP file from. STEP files are in plain text and a text editor can be used to examine the contents. For simple geometry they are easy to read.