I am a student learning English. I learned that 'a small fortune' means a large quantity of money. Then, is a small fortune the same as a large fortune? Is there no difference between them?
A small fortune is a relative term. It depends on the circumstances of the people involved. To someone impoverished and reduced to begging, £50 might be considered a small fortune. However this is a trivial sum to a well-to-do middle class person to whom a small fortune might mean £100,000. Whatever the sum, it usually implies a life changing amount.
On the other hand, a large fortune is a lot of money by almost any standard. The cost of a luxury yacht or a large country estate.
IMHO I disagree with M-W the sum doesn't have to be "surprising or unexpected". I don't consider any of the M-W examples meet this description . For example if you live in a draughty old house it is neither surprising nor unexpected that the heating bills will be large.
In modern English, a small fortune is a set phrase which has a very specific meaning. Merriam-Webster defines it as
a surprisingly or unexpectedly large amount of money
By contrast, a large fortune is not a set phrase- it means exactly what it says. A fortune is a large amount of money: a large fortune is even more than a fortune.