You can always right click on the top assembly in the left column, and set all to resolved. Lightweight just means that some or all of the parts are loaded as graphic bodies. This helps when you have large assemblies. If you don't want this to happen, then you either have to change the setting in System Options. Or when you Do a File>Open and you select your file, then at the bottom, it gives you the option of how you want to load the file. You can select 'Resolved' there if you want everything to be completely loaded in memory.