INSTALL IFC ADD-IN
New IFC add-ins for Revit are regularly being created and updated. Before doing an import or export make sure you are using the latest add-in.
There is an open source Add-in, also from other software vendors including; Graphisoft (ArchiCAD), Tekla, GeometryGym
(Note the Graphisoft Add-in will export Grids, others may not)
Otherwise search the Revit App store for IFC Add-ins.
IFC SIZE
After exporting run the IFC file through the free Solibri IFC Optimizer. This will dramatically reduce the file size.
You can also ZIP the IFC file reducing its size further.
CHECK THE IFC FILE
IT IS CRITICAL ALL IFC EXPORTS ARE CHECKED.
IFC exports are notorious for dropping elements.
Open the IFC export in an IFC viewer to check it visually. If you have IFC checking software use that.
Visually check everything you think is included is actually included.
IFC Viewers:
BIMsight (free)
IFC Checkers:
Solibri Model Checker (not free)
Or try this List of free IFC Software