Why do many plants produce such large fruits(apples and strawberries,for example) if those contain only relatively small 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.
Because seeds are not appealing to animals and they can only be carried by animals if they are made appealing - there are exceptions of course such as this one -. Seeds are covered with juicy layers that can be nutrient to animals in order to be made appealing and to be dispersed. Animals eat those juicy layers along with seeds, seeds resist digestion and fall on the ground where animals defecate. If the layer was thin and provided only protection to seeds, animals would not eat them and the seeds would have to be dispersed by other means. Therefore we would not call them fruits. The reason why seeds are not as large as their fruits is:
They have to be dry to resist climate changes while staying dormant below ground. And dry is generally small.
Seeds' task is to be dispersed, not to be nutritious to animals. And they usually do not taste very well compared to fruits - some do taste well as we know but I do not know why -. So why make them large and irritate the animal? Make them small so they will be overlooked by the animal and be dispersed happily
Most fruit/seed evolution is probably a reflection of the plants propagation strategy and any existing symbiotic forager relationship.
The strawberry expands its range by creeping with feeder shoots. It is also heavily foraged by small birds , thus the exterior small seeds.
Nut trees i.e. walnuts , are on the other end of the spectrum. The seed is nearly the size of the "fruit". All they want is some little squirrel to shake them loose and fall to the ground, then try to outgrow everything else.
Just making all this up.