Depends on what factors people need to explain that are not in the 'Personal checklist' generated at the 'Document Upload' stage.
Some people need to explain a lot (eg getting married or intending to get married, family members accompanying them to Canada, Travel History, Personal History factors, additional medical reports etc.), and some others need to explain almost nothing at all as it is covered in the rest of the documents they uploaded.
It varies from case to case. As long as you keep under the 4MB PDF limit it should be fine.