IFC file is a text file, that has IFC commands, describing building geometry. You can open IFC in text editor and actually read it. But because this is a script, automatically generated by program, it can be large, to compare any other 3D binary formats. Solibri optimiser removes unnecessary or repetitive data from the script, this way it reduce the file size. I don’t know how exactly Solibri optimise the file, but you can check by simple comparing original and optimised ifc in notepad. I can imagine, that original file can describes the layer name for each element, when during optimisation Solibri can set layer name once and after include into group all elements in this layer. Something like that. Maybe it also removes undefined properties.
