if you export an assembly into a catpart, all components will become independent solid bodies in a part. Surely you can use boolean operation to add them together to form ONE solid, provided that they are overlapping each other or have zero contact.
i am not sure whether we can change the resolution of wrl. But i am 100% sure that we can change this on STL. Change the 3d performance under Tools/options... (as i remember. i have no catia now)
if you cannot change the resolution, i suggest you can simplify your large model manually, e.g. delete small/unimportant/hidden details to make the file smaller beforeexportinginto wrl. Anyway, wrl is just for quickreview.