Let me give you some examples that will hopefully make it more clear:

"My house is as big as yours, but mine is probably bigger." = "My house is as big as yours, if not more so."

"I ate the same amount of food as you, but I probably ate more." = "I ate the same amount of food as you, if not more so.

So when you say 'if not more so', you are saying that there is a chance that the subject of your sentence is of a greater degree than you stated. Another example:

"This book weighs at least 100 pounds, but maybe it weighs even more." = "This book weight at least 100 pounds, if not more so."

I hope this helps!