There is a size limit of 700MB on IFC files uploaded to Bimsync. If you want to upload a file larger than 700MB you can try to zip it before uploading. Please use standard zip format and make sure that the zipped file contains only the ifc file. Next change the extension name of the file from .zip to .ifczip. Some authoring tools support Ifczip as an option when exporting IFC files. Example abc.ifczip
We do not recommend using the Solibri Optimizer for large Ifc files as these might get corrupted during the optimization process. Its better to zip tile file as explained. All information uploaded to Bimsync will be heavily optimized anyway so you will not see any gain by uploading an optimized files versus the original.