This would be a good research question for you. I recommend finding the entries for each of the three words here https://www.collinsdictionary.com/ and then deciding what you think. You could then check with us. On the second question, very few people can say why a word is similar to another word. It's like saying: why are lions and tigers similar?

Which of these two words "large" and "sufficient", best describes the meaning of the word "Substantial"? I agree with Michael. There is no shame in having to refer to a dictionary, even for proficient native speakers. Learning to use dictionaries, traditional or online will improve your English. Far more than asking questions whenever you get stuck.