In addition to selective breeding like others have mentioned, supply chain logistics have gotten much more advanced over the years. You can get many fruits right at the peak of ripeness year round due to sourcing and better storage methodologies.

Science has also gotten better at giving plants what they need to grow successfully, so almost all agricultural products are much larger than they would have been 50 years ago. If you take an apple tree from an orchard, and stick it in a random person's back yard and neglect it, it will have way smaller fruit. Irrigation, fertilization, etc, allow things to grow bigger, but the parts needed for the actual reproduction don't really grow much, so that extra energy just ends up producing fruit that's more "watered down".

In a grain, for example, theres 3 parts: germ, bran, and endosperm. The germ is the little start of the seedlings, and it contains protein, minerals, and fats. The bran is the other coating that has fiber, protein, and minerals. The endosperm is mostly just carbs. In modern grain, the endosperm takes up a much larger percentage of the grain than in older varieties (and non-fertilized/irrigated/weeded/pest controlled fields)