Why are fruits so large compared to their seeds?
The short answer: Fruits are large compared to seeds because humans have made them large. 

In the natural environment, there is a different set of evolutionary pressures. A fruit has to be able to successfully propagate itself using its seeds, while commercially farmed fruit is usually cloned via vegetative propagation. Therefore, the commercial farmed fruits do not need large seeds to propagate, since they are cloned by the farmers. In many cases, their seeds are actually nonviable (i.e. they will not grow when planted). 

For example, this is a wild banana, before being selectively bred by humans. As you can see here, the seeds are enormous compared to those in commercial Cavendish bananas. 



Similarly, this is a wild strawberry compared to its commercial farmed variant. 



Fruits with large seeds don't appeal to consumers, and therefore farmers who sell fruits with large seeds will get a poorer return on investment. 

Therefore, farmers who plant the fruits which appeal the best to consumers by being the easiest to eat (small seeds) will reap the greatest profits.